M2: implement vehicle return vertical slice

Transactional return command with idempotency, row-lock concurrency control, odometer-regression handling, vehicle status derivation, outbox event, audit trail. Result-summary UI on booking detail. 26 backend tests passing, ruff clean. Verified end-to-end via browser against S1 demo scenario; fixed two real defects found only through browser testing (UI state loss on status transition, unflushed UUID default).

backend/app/schemas.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 from datetime import datetime
 from typing import Any, Literal
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, conint
 
 Role = Literal["operations_manager", "rental_employee"]
 
@@ -48,6 +48,32 @@ class BookingOut(BookingSummaryOut):
     customer_name: str
 
 
+class RegisterReturnRequest(BaseModel):
+    end_odometer_km: conint(ge=0)
+    fuel_level_percent: conint(ge=0, le=100)
+    cleanliness_ok: bool
+    damage_reported: bool
+    technical_warning: bool
+    notes: str | None = Field(default=None, max_length=2000)
+
+
+class NextBookingRisk(BaseModel):
+    booking_ref: str
+    starts_at: datetime
+    at_risk: bool
+
+
+class RegisterReturnResult(BaseModel):
+    booking_ref: str
+    vehicle_ref: str
+    inspection_ref: str
+    resulting_vehicle_status: str
+    odometer_regression: bool
+    quality_issue_ref: str | None
+    workflow_event_id: str
+    next_booking_risk: NextBookingRisk | None
+
+
 class InspectionOut(BaseModel):
     public_ref: str
     booking_ref: str