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/pages/BookingDetail.tsx

@@ -1,24 +1,36 @@
-import { useEffect, useState } from "react";
+import { useCallback, useEffect, useState } from "react";
 import { Link, useParams } from "react-router-dom";
 import { api } from "../api/client";
-import type { Booking } from "../api/types";
+import type { Booking, RegisterReturnResult } from "../api/types";
 import { StatusBadge } from "../components/Badge";
+import { ReturnForm, ReturnResultPanel } from "../components/ReturnForm";
 
 export function BookingDetail() {
   const { publicRef } = useParams<{ publicRef: string }>();
   const [booking, setBooking] = useState<Booking | null>(null);
   const [error, setError] = useState<string | null>(null);
+  const [returnResult, setReturnResult] = useState<RegisterReturnResult | null>(null);
 
-  useEffect(() => {
+  const load = useCallback(() => {
     if (!publicRef) return;
-    setBooking(null);
-    setError(null);
     api
       .get<Booking>(`/api/v1/bookings/${publicRef}`)
       .then(setBooking)
       .catch(() => setError("This booking could not be found."));
   }, [publicRef]);
 
+  useEffect(() => {
+    setBooking(null);
+    setError(null);
+    setReturnResult(null);
+    load();
+  }, [load]);
+
+  function handleRegistered(result: RegisterReturnResult) {
+    setReturnResult(result);
+    load();
+  }
+
   if (error) return <p className="error" role="alert">{error}</p>;
   if (!booking) return <p>Loading booking…</p>;
 
@@ -36,6 +48,11 @@ export function BookingDetail() {
         <div><dt>End odometer</dt><dd>{booking.end_odometer_km ?? "—"} km</dd></div>
         <div><dt>Requirements complete</dt><dd>{booking.requirements_complete ? "Yes" : "No"}</dd></div>
       </dl>
+
+      {returnResult && <ReturnResultPanel result={returnResult} />}
+      {!returnResult && booking.status === "active" && (
+        <ReturnForm bookingRef={booking.public_ref} onRegistered={handleRegistered} />
+      )}
     </div>
   );
 }