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/tests/test_seed.py

@@ -7,11 +7,16 @@ from app.models.data_quality import DataQualityIssue
 from app.models.outbox import OutboxEvent
 from app.models.user import User
 from app.models.vehicle import Vehicle
+from app.seed_loader import reset_and_seed
 
 
 def test_seed_counts_match_deterministic_dataset():
+    # Other test modules mutate shared demo state (returns, resets), so this test
+    # re-seeds immediately before asserting counts rather than trusting whatever
+    # order pytest happened to run modules in.
     db = SessionLocal()
     try:
+        reset_and_seed(db)
         assert db.scalar(select(func.count()).select_from(Vehicle)) == 50
         assert db.scalar(select(func.count()).select_from(Customer)) == 180
         assert db.scalar(select(func.count()).select_from(Booking)) == 246
@@ -25,6 +30,8 @@ def test_seed_counts_match_deterministic_dataset():
 def test_seed_demo_scenarios_present():
     db = SessionLocal()
     try:
+        reset_and_seed(db)
+
         booking = db.scalar(select(Booking).where(Booking.public_ref == "BK-DEMO-RETURN"))
         assert booking is not None
         assert booking.status == "active"