feat(returns): add authoritative return preview

The return-review step predicted operational consequences independently in
the frontend, and got it wrong: damage or a technical warning was described
as routing to "maintenance" when the actual domain rule (returns.py) routes
it to "blocked", and the no-contradiction case was described as becoming
"available" when the vehicle actually always goes to "cleaning" first
(only reaching "maintenance" if the service threshold was crossed).

Extract the evaluation returns.py already performed inline into a pure
evaluate_return() function with no writes -- resulting status (with an
explanation), odometer regression, would-create-quality-issue,
next-booking-risk -- and share it between a new non-mutating
POST /bookings/{ref}/return-preview endpoint and the existing commit path,
so preview and commit can never drift apart again. The result screen also
now distinguishes local commit success from n8n delivery (still queued/
unconfirmed) instead of implying both succeeded, and links to any created
quality issue for Operations Manager.

backend/app/api/routers/bookings.py

@@ -8,8 +8,14 @@ from app.api.deps import get_current_user, get_db
 from app.models.booking import Booking
 from app.models.customer import Customer
 from app.models.vehicle import Vehicle
-from app.schemas import BookingOut, CurrentUser, RegisterReturnRequest
-from app.services.returns import register_vehicle_return
+from app.schemas import (
+    BookingOut,
+    CurrentUser,
+    NextBookingRisk,
+    RegisterReturnRequest,
+    ReturnPreviewResult,
+)
+from app.services.returns import preview_vehicle_return, register_vehicle_return
 
 router = APIRouter(prefix="/api/v1/bookings", tags=["bookings"])
 
@@ -66,6 +72,33 @@ def get_booking(
     return _to_out(booking, customer, vehicle)
 
 
+@router.post("/{public_ref}/return-preview", response_model=ReturnPreviewResult)
+def preview_return(
+    public_ref: str,
+    body: RegisterReturnRequest,
+    db: Session = Depends(get_db),
+    _user: CurrentUser = Depends(get_current_user),
+) -> ReturnPreviewResult:
+    booking, vehicle, evaluation = preview_vehicle_return(db, public_ref, body)
+    return ReturnPreviewResult(
+        booking_ref=booking.public_ref,
+        vehicle_ref=vehicle.public_ref,
+        canonical_odometer_km=evaluation.canonical_odometer_km,
+        submitted_odometer_km=evaluation.submitted_odometer_km,
+        odometer_regression=evaluation.odometer_regression,
+        resulting_odometer_km=evaluation.resulting_odometer_km,
+        resulting_vehicle_status=evaluation.resulting_vehicle_status,
+        status_reason=evaluation.status_reason,
+        would_create_quality_issue=evaluation.would_create_quality_issue,
+        attention_reasons=evaluation.attention_reasons,
+        next_booking_risk=(
+            NextBookingRisk(**evaluation.next_booking_risk)
+            if evaluation.next_booking_risk is not None
+            else None
+        ),
+    )
+
+
 @router.post("/{public_ref}/return")
 def register_return(
     public_ref: str,

backend/app/schemas.py

@@ -74,6 +74,20 @@ class RegisterReturnResult(BaseModel):
     next_booking_risk: NextBookingRisk | None
 
 
+class ReturnPreviewResult(BaseModel):
+    booking_ref: str
+    vehicle_ref: str
+    canonical_odometer_km: int
+    submitted_odometer_km: int
+    odometer_regression: bool
+    resulting_odometer_km: int
+    resulting_vehicle_status: str
+    status_reason: str
+    would_create_quality_issue: bool
+    attention_reasons: list[str]
+    next_booking_risk: NextBookingRisk | None
+
+
 class InspectionOut(BaseModel):
     public_ref: str
     booking_ref: str
@@ -216,6 +230,10 @@ class AuditEventOut(BaseModel):
     action: str
     entity_type: str
     entity_id: str | None
+    entity_ref: str | None = None
+    entity_link: str | None = None
     correlation_id: str
     occurred_at: datetime
+    before: dict[str, Any] | None = None
+    after: dict[str, Any] | None = None
     metadata: dict[str, Any] | None = None

backend/app/services/returns.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import uuid
+from dataclasses import dataclass
 from datetime import UTC, datetime
 
 from sqlalchemy import select
@@ -25,12 +26,120 @@ def _next_public_ref(db: Session) -> str:
     return f"{REF_PREFIX}-{len(existing) + 1:04d}"
 
 
-def _derive_vehicle_status(body: RegisterReturnRequest, vehicle: Vehicle, new_odometer: int) -> str:
-    if body.damage_reported or body.technical_warning:
-        return "blocked"
+def _derive_vehicle_status_with_reason(
+    body: RegisterReturnRequest, vehicle: Vehicle, new_odometer: int
+) -> tuple[str, str]:
+    if body.damage_reported and body.technical_warning:
+        return "blocked", "Damage and a technical warning were both reported on return."
+    if body.damage_reported:
+        return "blocked", "Damage was reported on return."
+    if body.technical_warning:
+        return "blocked", "A technical warning was reported on return."
     if new_odometer >= vehicle.next_service_km:
-        return "maintenance"
-    return "cleaning"
+        return (
+            "maintenance",
+            f"Odometer reached the {vehicle.next_service_km:,} km service threshold.",
+        )
+    return "cleaning", "No damage, technical warning or service threshold; routed to cleaning."
+
+
+@dataclass
+class ReturnEvaluation:
+    canonical_odometer_km: int
+    submitted_odometer_km: int
+    odometer_regression: bool
+    resulting_odometer_km: int
+    resulting_vehicle_status: str
+    status_reason: str
+    would_create_quality_issue: bool
+    attention_reasons: list[str]
+    next_booking_risk: dict | None
+
+
+def evaluate_return(
+    db: Session, booking: Booking, vehicle: Vehicle, body: RegisterReturnRequest, *, now: datetime
+) -> ReturnEvaluation:
+    """Pure evaluation of what a return would do. No writes; safe to call from a
+    non-mutating preview endpoint. `register_vehicle_return` uses the same function so
+    preview and commit can never drift apart."""
+    odometer_regression = body.end_odometer_km < vehicle.odometer_km
+    resulting_odometer_km = vehicle.odometer_km if odometer_regression else body.end_odometer_km
+    resulting_status, status_reason = _derive_vehicle_status_with_reason(
+        body, vehicle, resulting_odometer_km
+    )
+
+    attention_reasons = []
+    if body.damage_reported:
+        attention_reasons.append("damage_reported")
+    if body.technical_warning:
+        attention_reasons.append("technical_warning")
+    if odometer_regression:
+        attention_reasons.append("odometer_regression")
+
+    next_booking = db.scalar(
+        select(Booking)
+        .where(
+            Booking.vehicle_id == vehicle.id,
+            Booking.status == "reserved",
+            Booking.starts_at > now,
+        )
+        .order_by(Booking.starts_at.asc())
+    )
+    next_booking_risk = None
+    if next_booking is not None:
+        hours_until = (next_booking.starts_at - now).total_seconds() / 3600
+        next_booking_risk = {
+            "booking_ref": next_booking.public_ref,
+            "starts_at": next_booking.starts_at.isoformat(),
+            "at_risk": resulting_status != "cleaning" or hours_until < 4,
+        }
+
+    return ReturnEvaluation(
+        canonical_odometer_km=vehicle.odometer_km,
+        submitted_odometer_km=body.end_odometer_km,
+        odometer_regression=odometer_regression,
+        resulting_odometer_km=resulting_odometer_km,
+        resulting_vehicle_status=resulting_status,
+        status_reason=status_reason,
+        would_create_quality_issue=odometer_regression,
+        attention_reasons=attention_reasons,
+        next_booking_risk=next_booking_risk,
+    )
+
+
+def _load_active_booking_and_vehicle(
+    db: Session, booking_ref: str, *, lock: bool
+) -> tuple[Booking, Vehicle]:
+    stmt = select(Booking).where(Booking.public_ref == booking_ref)
+    if lock:
+        stmt = stmt.with_for_update()
+    booking = db.scalar(stmt)
+    if booking is None:
+        raise AppError("BOOKING_NOT_FOUND", "Booking not found.", status_code=404)
+
+    vehicle_stmt = select(Vehicle).where(Vehicle.id == booking.vehicle_id)
+    if lock:
+        vehicle_stmt = vehicle_stmt.with_for_update()
+    vehicle = db.scalar(vehicle_stmt)
+    if vehicle is None:
+        raise AppError(
+            "VEHICLE_NOT_FOUND", "The vehicle for this booking could not be found.", status_code=404
+        )
+    return booking, vehicle
+
+
+def preview_vehicle_return(
+    db: Session, booking_ref: str, body: RegisterReturnRequest
+) -> tuple[Booking, Vehicle, ReturnEvaluation]:
+    booking, vehicle = _load_active_booking_and_vehicle(db, booking_ref, lock=False)
+    if booking.status != "active":
+        raise AppError(
+            "INVALID_BOOKING_STATE",
+            f"Booking is '{booking.status}', not 'active'; it cannot be returned.",
+            status_code=409,
+        )
+    evaluation = evaluate_return(db, booking, vehicle, body, now=datetime.now(UTC))
+    return booking, vehicle, evaluation
 
 
 def register_vehicle_return(
@@ -53,14 +162,7 @@ def register_vehicle_return(
             )
         return existing.response_status, existing.response_body
 
-    booking = db.scalar(select(Booking).where(Booking.public_ref == booking_ref).with_for_update())
-    if booking is None:
-        raise AppError("BOOKING_NOT_FOUND", "Booking not found.", status_code=404)
-    vehicle = db.scalar(select(Vehicle).where(Vehicle.id == booking.vehicle_id).with_for_update())
-    if vehicle is None:
-        raise AppError(
-            "VEHICLE_NOT_FOUND", "The vehicle for this booking could not be found.", status_code=404
-        )
+    booking, vehicle = _load_active_booking_and_vehicle(db, booking_ref, lock=True)
 
     # Re-check after acquiring the row lock: a concurrent identical-key request may have
     # just committed while we were waiting.
@@ -79,6 +181,7 @@ def register_vehicle_return(
 
     now = datetime.now(UTC)
     correlation_id = uuid.uuid4()
+    evaluation = evaluate_return(db, booking, vehicle, body, now=now)
 
     inspection = Inspection(
         public_ref=_next_public_ref(db),
@@ -104,13 +207,8 @@ def register_vehicle_return(
     booking.status = "returned"
     booking.end_odometer_km = body.end_odometer_km
 
-    odometer_regression = body.end_odometer_km < vehicle.odometer_km
     quality_issue_ref: str | None = None
-    canonical_odometer = vehicle.odometer_km
-    if not odometer_regression:
-        canonical_odometer = body.end_odometer_km
-        vehicle.odometer_km = canonical_odometer
-    else:
+    if evaluation.odometer_regression:
         issue = DataQualityIssue(
             public_ref=f"DQ-RET-{str(inspection.public_ref).split('-')[-1]}",
             rule_type="odometer_regression",
@@ -121,7 +219,7 @@ def register_vehicle_return(
             evidence_json={
                 "summary": (
                     f"Return submitted {body.end_odometer_km} km, below canonical "
-                    f"{vehicle.odometer_km} km."
+                    f"{evaluation.canonical_odometer_km} km."
                 ),
                 "entity_ref": vehicle.public_ref,
                 "related_refs": [booking.public_ref, inspection.public_ref],
@@ -133,7 +231,8 @@ def register_vehicle_return(
         db.flush()
         quality_issue_ref = issue.public_ref
 
-    resulting_status = _derive_vehicle_status(body, vehicle, canonical_odometer)
+    resulting_status = evaluation.resulting_vehicle_status
+    vehicle.odometer_km = evaluation.resulting_odometer_km
     vehicle.operational_status = resulting_status
     vehicle.version += 1
 
@@ -164,14 +263,6 @@ def register_vehicle_return(
         },
     )
 
-    attention_reasons = []
-    if body.damage_reported:
-        attention_reasons.append("damage_reported")
-    if body.technical_warning:
-        attention_reasons.append("technical_warning")
-    if odometer_regression:
-        attention_reasons.append("odometer_regression")
-
     event = OutboxEvent(
         event_id=uuid.uuid4(),
         event_type="vehicle.returned.v1",
@@ -189,7 +280,7 @@ def register_vehicle_return(
                 "vehicle_ref": vehicle.public_ref,
                 "inspection_ref": inspection.public_ref,
                 "resulting_vehicle_status": resulting_status,
-                "attention_reasons": attention_reasons,
+                "attention_reasons": evaluation.attention_reasons,
             },
             "aggregate_ref": booking.public_ref,
         },
@@ -199,33 +290,15 @@ def register_vehicle_return(
     )
     db.add(event)
 
-    next_booking = db.scalar(
-        select(Booking)
-        .where(
-            Booking.vehicle_id == vehicle.id,
-            Booking.status == "reserved",
-            Booking.starts_at > now,
-        )
-        .order_by(Booking.starts_at.asc())
-    )
-    next_booking_risk = None
-    if next_booking is not None:
-        hours_until = (next_booking.starts_at - now).total_seconds() / 3600
-        next_booking_risk = {
-            "booking_ref": next_booking.public_ref,
-            "starts_at": next_booking.starts_at.isoformat(),
-            "at_risk": resulting_status != "cleaning" or hours_until < 4,
-        }
-
     response_body = {
         "booking_ref": booking.public_ref,
         "vehicle_ref": vehicle.public_ref,
         "inspection_ref": inspection.public_ref,
         "resulting_vehicle_status": resulting_status,
-        "odometer_regression": odometer_regression,
+        "odometer_regression": evaluation.odometer_regression,
         "quality_issue_ref": quality_issue_ref,
         "workflow_event_id": str(event.event_id),
-        "next_booking_risk": next_booking_risk,
+        "next_booking_risk": evaluation.next_booking_risk,
     }
 
     db.add(

backend/tests/test_return.py

@@ -1,11 +1,16 @@
 import threading
+import uuid
+from datetime import UTC, datetime, timedelta
 
 from fastapi.testclient import TestClient
 from sqlalchemy import select
 
 from app.core.db import SessionLocal
 from app.main import app
+from app.models.audit import AuditEvent
 from app.models.booking import Booking
+from app.models.customer import Customer
+from app.models.outbox import OutboxEvent
 from app.models.vehicle import Vehicle
 
 
@@ -39,6 +44,148 @@ def _activate_booking(vehicle_ref: str, start_odometer_km: int) -> str:
         db.close()
 
 
+def _set_next_service_km(vehicle_ref: str, threshold: int) -> None:
+    db = SessionLocal()
+    try:
+        vehicle = db.scalar(select(Vehicle).where(Vehicle.public_ref == vehicle_ref))
+        vehicle.next_service_km = threshold
+        db.commit()
+    finally:
+        db.close()
+
+
+def _add_reserved_booking(vehicle_ref: str, *, hours_from_now: float) -> str:
+    db = SessionLocal()
+    try:
+        vehicle = db.scalar(select(Vehicle).where(Vehicle.public_ref == vehicle_ref))
+        customer = db.scalar(select(Customer))
+        starts_at = datetime.now(UTC) + timedelta(hours=hours_from_now)
+        public_ref = f"BK-TEST-{uuid.uuid4().hex[:8].upper()}"
+        booking = Booking(
+            public_ref=public_ref,
+            customer_id=customer.id,
+            vehicle_id=vehicle.id,
+            starts_at=starts_at,
+            ends_at=starts_at + timedelta(days=2),
+            status="reserved",
+            requirements_complete=True,
+        )
+        db.add(booking)
+        db.commit()
+        return public_ref
+    finally:
+        db.close()
+
+
+def _counts() -> tuple[int, int]:
+    db = SessionLocal()
+    try:
+        return (
+            len(db.scalars(select(AuditEvent)).all()),
+            len(db.scalars(select(OutboxEvent)).all()),
+        )
+    finally:
+        db.close()
+
+
+def test_preview_performs_no_writes_and_matches_commit(ops_client):
+    booking_ref = _activate_booking("MO-006", start_odometer_km=30000)
+    vehicle_before = ops_client.get("/api/v1/vehicles/MO-006").json()
+    new_reading = vehicle_before["odometer_km"] + 25
+    body = _return_body(end_odometer_km=new_reading)
+
+    audit_before, outbox_before = _counts()
+    preview = ops_client.post(f"/api/v1/bookings/{booking_ref}/return-preview", json=body)
+    assert preview.status_code == 200
+    preview_body = preview.json()
+    audit_after, outbox_after = _counts()
+    assert (audit_after, outbox_after) == (audit_before, outbox_before)
+
+    booking_mid = ops_client.get(f"/api/v1/bookings/{booking_ref}").json()
+    assert booking_mid["status"] == "active"  # preview did not mutate the booking
+    vehicle_mid = ops_client.get("/api/v1/vehicles/MO-006").json()
+    assert vehicle_mid["odometer_km"] == vehicle_before["odometer_km"]
+
+    assert preview_body["odometer_regression"] is False
+    assert preview_body["resulting_odometer_km"] == new_reading
+    assert preview_body["canonical_odometer_km"] == vehicle_before["odometer_km"]
+
+    commit = ops_client.post(
+        f"/api/v1/bookings/{booking_ref}/return",
+        json=body,
+        headers={"Idempotency-Key": "test-preview-matches-commit-001"},
+    )
+    assert commit.status_code == 201
+    commit_body = commit.json()
+    assert commit_body["resulting_vehicle_status"] == preview_body["resulting_vehicle_status"]
+    assert commit_body["odometer_regression"] == preview_body["odometer_regression"]
+    assert commit_body["next_booking_risk"] == preview_body["next_booking_risk"]
+
+
+def test_preview_detects_odometer_regression(ops_client):
+    booking_ref = _activate_booking("MO-007", start_odometer_km=15000)
+    vehicle_before = ops_client.get("/api/v1/vehicles/MO-007").json()
+    low_reading = vehicle_before["odometer_km"] - 100
+
+    preview = ops_client.post(
+        f"/api/v1/bookings/{booking_ref}/return-preview",
+        json=_return_body(end_odometer_km=low_reading),
+    )
+    assert preview.status_code == 200
+    body = preview.json()
+    assert body["odometer_regression"] is True
+    assert body["would_create_quality_issue"] is True
+    assert "odometer_regression" in body["attention_reasons"]
+    assert body["resulting_odometer_km"] == vehicle_before["odometer_km"]
+
+    vehicle_after = ops_client.get("/api/v1/vehicles/MO-007").json()
+    assert vehicle_after["odometer_km"] == vehicle_before["odometer_km"]
+
+
+def test_preview_detects_service_due(ops_client):
+    booking_ref = _activate_booking("MO-009", start_odometer_km=18000)
+    vehicle_before = ops_client.get("/api/v1/vehicles/MO-009").json()
+    _set_next_service_km("MO-009", vehicle_before["odometer_km"] + 50)
+
+    preview = ops_client.post(
+        f"/api/v1/bookings/{booking_ref}/return-preview",
+        json=_return_body(end_odometer_km=vehicle_before["odometer_km"] + 100),
+    )
+    assert preview.status_code == 200
+    body = preview.json()
+    assert body["resulting_vehicle_status"] == "maintenance"
+    assert "service threshold" in body["status_reason"]
+
+
+def test_preview_detects_next_booking_risk(ops_client):
+    booking_ref = _activate_booking("MO-011", start_odometer_km=19000)
+    _add_reserved_booking("MO-011", hours_from_now=2)
+
+    preview = ops_client.post(
+        f"/api/v1/bookings/{booking_ref}/return-preview",
+        json=_return_body(end_odometer_km=19500),
+    )
+    assert preview.status_code == 200
+    risk = preview.json()["next_booking_risk"]
+    assert risk is not None
+    assert risk["at_risk"] is True  # less than 4 hours away
+
+
+def test_preview_requires_active_booking(ops_client):
+    booking_ref = _activate_booking("MO-014", start_odometer_km=21000)
+    ops_client.post(
+        f"/api/v1/bookings/{booking_ref}/return",
+        json=_return_body(end_odometer_km=21500),
+        headers={"Idempotency-Key": "test-preview-requires-active-001"},
+    )
+    preview = ops_client.post(
+        f"/api/v1/bookings/{booking_ref}/return-preview",
+        json=_return_body(end_odometer_km=22000),
+    )
+    assert preview.status_code == 409
+    assert preview.json()["error"]["code"] == "INVALID_BOOKING_STATE"
+
+
 def test_register_return_success_updates_canonical_odometer(ops_client):
     booking_ref = _activate_booking("MO-003", start_odometer_km=20000)
     vehicle_before = ops_client.get("/api/v1/vehicles/MO-003").json()