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).

frontend/src/api/types.ts

@@ -118,6 +118,32 @@ export interface Dashboard {
   recent_automation: AutomationRun[];
 }
 
+export interface RegisterReturnRequest {
+  end_odometer_km: number;
+  fuel_level_percent: number;
+  cleanliness_ok: boolean;
+  damage_reported: boolean;
+  technical_warning: boolean;
+  notes?: string;
+}
+
+export interface NextBookingRisk {
+  booking_ref: string;
+  starts_at: string;
+  at_risk: boolean;
+}
+
+export interface RegisterReturnResult {
+  booking_ref: string;
+  vehicle_ref: string;
+  inspection_ref: string;
+  resulting_vehicle_status: string;
+  odometer_regression: boolean;
+  quality_issue_ref: string | null;
+  workflow_event_id: string;
+  next_booking_risk: NextBookingRisk | null;
+}
+
 export interface AuditEvent {
   id: string;
   actor_type: string;