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/components/Layout.tsx

@@ -6,6 +6,7 @@ const NAV_ITEMS = [
   { to: "/vehicles", label: "Vehicles" },
   { to: "/bookings", label: "Bookings" },
   { to: "/data-quality", label: "Data Quality" },
+  { to: "/automation", label: "Automation" },
   { to: "/audit", label: "Audit" },
 ];