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).

PROJECT_STATE.md

@@ -2,7 +2,7 @@
 
 ## Current milestone
 
-M1 — complete. Starting M2 next.
+M2 — complete. Starting M3 next.
 
 ## Locked decisions
 
@@ -22,6 +22,12 @@ M1 — complete. Starting M2 next.
 - `compose.yaml` api build context changed from `./backend` to repo root with `dockerfile: backend/Dockerfile`, so the image can `COPY seed ./seed` (seed CSVs are outside `backend/`).
 - Frontend: added `react-router-dom@7.18.2` (bumped from 6.x to clear two real advisories — open redirect + arbitrary constructor injection in v6). One residual `npm audit` finding (RSC-mode CSRF, GHSA-qwww-vcr4-c8h2) does not apply — this SPA never uses React Router's RSC/SSR mode.
 - Nav/pages built so far: Dashboard, Vehicles (list+detail with tabs), Bookings (list+detail), Audit. Data Quality, Knowledge and Automation nav items are intentionally omitted until M3/M5/M4 build the pages behind them — CLAUDE.md forbids dead routes/placeholders.
+- Return workflow (`app/services/returns.py`): the spec's "validate submitted reading against booking start reading" step was dropped as a hard rejection. For the seeded S1 scenario, a booking's `start_odometer_km` can already equal the vehicle's canonical odometer, so any regression-testing value would also be below the booking start, making a hard floor there indistinguishable from — and in conflict with — the documented soft-regression path. Only one odometer check exists now: submitted vs. the vehicle's *canonical* odometer (`vehicle.odometer_km`), matching the domain-model invariant verbatim ("a return with a lower submitted reading is recorded as an inspection and issue, while canonical odometer remains unchanged").
+- Idempotency: new `idempotency_records` table (migration `e7b08389f47f`), unique on `idempotency_key`, keyed to `booking_id`. Same key + same booking replays the stored response; same key + different booking → 409 `IDEMPOTENCY_KEY_REUSED`; different key on an already-returned booking → 409 `INVALID_BOOKING_STATE`. Concurrency is enforced by `SELECT ... FOR UPDATE` on the booking row (re-checked for the idempotency record immediately after acquiring the lock, as a safety net for two simultaneous identical-key requests racing the pre-lock check).
+- `seed_loader.clear_all()` must delete `idempotency_records` before `bookings` (FK) — easy to forget when adding new booking-referencing tables; the ordering list at the top of `seed_loader.py` is the single place to update.
+- Inspection `public_ref` is assigned as `INSP-{count+1:04d}` from a live count query (not gap-safe, fine for a PoC single-writer demo, would need a sequence for real concurrency-safe numbering).
+- Found and fixed during browser verification (not caught by pytest, since it's a UI-only defect): `ReturnForm` originally held its own `result` state and was conditionally rendered only when `booking.status === "active"`; once the return succeeded the booking flipped to `returned` and React unmounted the form before the user ever saw the result panel. Fixed by lifting the result into `BookingDetail` (`ReturnResultPanel` is now a sibling, not nested in `ReturnForm`). Also found: `OutboxEvent.event_id`'s Python-side `default=uuid.uuid4` on the mapped_column only applies at flush/commit time, so reading `event.event_id` before `db.commit()` returned `None` (rendered as the literal string "None" in the result panel); fixed by assigning `event_id=uuid.uuid4()` explicitly at construction. Lesson: SQLAlchemy column `default=` callables are not available on the in-memory Python object until flush — never rely on the generated value for a same-transaction response body without an explicit `db.flush()` or an explicit Python-side assignment.
+- Operational note for this environment: `docker compose run --rm api ...` (used for tests/lint) only starts a throwaway one-off container — it does **not** update the long-running `api`/`web` service containers. After any code change meant to be verified live (browser, curl), `docker compose up -d --build <service>` is required, not just `docker compose build`.
 
 ## Completed evidence
 
@@ -49,10 +55,20 @@ M1 — complete. Starting M2 next.
   - Browser smoke test (Chrome via MCP) at desktop width: login page → Operations Manager login → Dashboard (metrics + attention items + today + recent automation all populated) → Vehicle detail `MO-016` (tabs render, "Needs attention" badge correct — it's `DQ-DEMO-OVERLAP`/`DQ-DEMO-STATUS`) → Booking detail `BK-DEMO-RETURN` (matches S1 scenario: vehicle `MO-024`, status `active`, start odometer `53610`). Responsive CSS (`@media max-width:700px`) was written and code-reviewed but the automated resize during this session didn't visibly reflect in the captured screenshot (likely a screenshot-timing quirk of the browser tool, not necessarily a real bug) — **treat the ≤360px layout as visually unverified** and re-check with a real device/DevTools emulation before final acceptance (M7).
 - Known accepted gap carried over from M0: `npm audit` residual `esbuild`/Vite-8 dev-server-only advisory.
 
+### M2 — Vehicle return vertical slice
+- Backend additions: `app/models/idempotency.py` (`IdempotencyRecord`), migration `e7b08389f47f_idempotency_records`, `app/services/returns.py` (`register_vehicle_return` — full transaction: row locks, idempotency replay, inspection, canonical-odometer update or regression issue, vehicle status derivation, two audit events, `vehicle.returned.v1` outbox event matching `contracts/events.schema.json`, next-booking-risk lookup), `POST /api/v1/bookings/{public_ref}/return` wired in `app/api/routers/bookings.py` with required `Idempotency-Key` header.
+- Frontend additions: `components/ReturnForm.tsx` (form + `ReturnResultPanel`), wired into `pages/BookingDetail.tsx` (shown only when `booking.status === "active"`; result persists via lifted state after the booking flips to `returned`).
+- Commands run and verified from this checkout:
+  - `docker compose run --rm api pytest -q` — **26 passed**, including `tests/test_return.py` (success/canonical-update, S1 regression scenario by name, damage→blocked, idempotent replay, reject-already-returned, missing-header validation, and a real multi-threaded concurrent-submission test against Postgres asserting exactly 1×201 + 2×409).
+  - `docker compose run --rm api ruff check .` — All checks passed.
+  - `npm run build` — clean.
+  - `docker compose up -d --build` (all services) then `docker compose exec api python -m app.cli seed --reset`, then a full browser run of the S1 demo scenario against `BK-DEMO-RETURN`/`MO-024`: submitted 53000 km (below canonical 54820) → result panel showed `INSP-0076`, `resulting_vehicle_status: maintenance` (correctly derived, since canonical 54820 ≥ `next_service_km` 40000), `DQ-RET-0076` created, automation event queued with a real UUID, "no upcoming booking" risk; vehicle detail page confirmed odometer unchanged at 54,820 km and a "Needs attention" badge.
+  - Both real defects listed above (form disappearing before showing its result; `event_id` reading as `None`) were **found via the browser run, not by pytest** — the test suite asserted on API response shape/values, not on what the UI actually rendered after a status transition. Worth remembering for M3+: UI state-after-mutation bugs need a browser check, not just API tests.
+
 ## Known blockers
 
 None. External service credentials may be absent; use the documented demo/degraded providers.
 
 ## Exact next action
 
-Start M2 (vehicle return vertical slice): read `docs/08-return-workflow.md`, `contracts/events.schema.json`. Implement `POST /api/v1/bookings/{public_ref}/return` per the transaction steps in that doc (lock booking+vehicle rows, idempotency by header key, odometer-regression handling, status derivation, quality-issue creation, outbox insert, audit, one commit), a result-summary UI on the booking page, and tests for success/regression/replay/concurrent-submission/rollback. The seeded `BK-DEMO-RETURN` (`MO-024`, currently `active`, start odometer 53610) is the scripted demo scenario (S1) — a return below 53610 should trigger the regression path.
+Start M3 (Data Quality Workbench): read `docs/07-data-quality.md`. Implement the five rule scanners (possible_duplicate_customer with weighted scoring, missing_required_field, odometer_regression, booking_overlap, vehicle_status_conflict) as an explicit scan service (idempotent by `(rule_type, entity_type, entity_id, evidence fingerprint)` while open, per the doc's lifecycle section), issue defer/reject/merge-customers endpoints, a transactional customer-merge (rewires bookings, tombstones the loser, audits before/after), the Data Quality nav item + Workbench UI (two-column duplicate comparison, evidence/proposed-resolution for other types), and wire "open quality issues" into the dashboard attention section it already reads from. Seed data already has 15 `data_quality_issues` including the named `DQ-DEMO-*` scenarios (S2 duplicate CUS-0012/CUS-0178, S4 overlap MO-016) to scan/resolve against — remember M2's return workflow also creates ad-hoc `odometer_regression` issues (`DQ-RET-*`) directly, so the scan service's idempotent-fingerprint rule must not double-report those.