M4: implement n8n automation

Outbox dispatcher (background thread, FOR UPDATE SKIP LOCKED claim, exponential backoff, no transaction held during HTTP I/O). n8n callback endpoint with shared-secret auth and idempotency by event ID. Automation nav + UI with manual retry. 49 backend tests passing, ruff clean. Fixed a crash-on-redelivery bug in seeded outbox payloads and made the dispatcher defensive against malformed payloads. Verified the full live round trip against a real n8n instance: return -> outbox -> dispatcher -> n8n workflow -> callback -> succeeded, including the S5 failed-retry demo scenario.

frontend/src/App.tsx

@@ -10,6 +10,7 @@ import { Bookings } from "./pages/Bookings";
 import { BookingDetail } from "./pages/BookingDetail";
 import { DataQuality } from "./pages/DataQuality";
 import { DataQualityIssueDetail } from "./pages/DataQualityIssueDetail";
+import { Automation } from "./pages/Automation";
 import { Audit } from "./pages/Audit";
 
 export function App() {
@@ -31,6 +32,7 @@ export function App() {
           <Route path="/bookings/:publicRef" element={<BookingDetail />} />
           <Route path="/data-quality" element={<DataQuality />} />
           <Route path="/data-quality/:publicRef" element={<DataQualityIssueDetail />} />
+          <Route path="/automation" element={<Automation />} />
           <Route path="/audit" element={<Audit />} />
         </Route>
         <Route path="/" element={<Navigate to="/dashboard" replace />} />