M3: implement Data Quality Workbench

Five rule scanners (duplicate customers, missing fields, odometer regression, booking overlap, status conflict) run automatically after seed and via an explicit scan endpoint. Issue defer/reject/merge-customers endpoints with transactional customer merge (booking rewiring, tombstone, audit). Data Quality nav + workbench UI with two-column duplicate comparison and inline (non-native) confirm. Dashboard attention items now link to issues. 35 backend tests passing, ruff clean. Fixed a real false-positive bug in odometer-regression detection found through iteration on seed data, and two TS narrowing errors. Verified end-to-end via browser: S2 merge and S4 overlap scenarios.

frontend/src/App.tsx

@@ -8,6 +8,8 @@ import { Vehicles } from "./pages/Vehicles";
 import { VehicleDetail } from "./pages/VehicleDetail";
 import { Bookings } from "./pages/Bookings";
 import { BookingDetail } from "./pages/BookingDetail";
+import { DataQuality } from "./pages/DataQuality";
+import { DataQualityIssueDetail } from "./pages/DataQualityIssueDetail";
 import { Audit } from "./pages/Audit";
 
 export function App() {
@@ -27,6 +29,8 @@ export function App() {
           <Route path="/vehicles/:publicRef" element={<VehicleDetail />} />
           <Route path="/bookings" element={<Bookings />} />
           <Route path="/bookings/:publicRef" element={<BookingDetail />} />
+          <Route path="/data-quality" element={<DataQuality />} />
+          <Route path="/data-quality/:publicRef" element={<DataQualityIssueDetail />} />
           <Route path="/audit" element={<Audit />} />
         </Route>
         <Route path="/" element={<Navigate to="/dashboard" replace />} />