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/models/__init__.py

@@ -3,6 +3,7 @@ from app.models.audit import AuditEvent
 from app.models.booking import Booking
 from app.models.customer import Customer
 from app.models.data_quality import DataQualityIssue
+from app.models.idempotency import IdempotencyRecord
 from app.models.inspection import Inspection
 from app.models.maintenance import MaintenanceRecord
 from app.models.outbox import OutboxEvent
@@ -15,6 +16,7 @@ __all__ = [
     "Booking",
     "Customer",
     "DataQualityIssue",
+    "IdempotencyRecord",
     "Inspection",
     "MaintenanceRecord",
     "OutboxEvent",