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.

n8n/mobilityops-return-processing.json

@@ -1,4 +1,5 @@
 {
+  "id": "mobilityops-return-processing",
   "name": "MobilityOps - Vehicle Return Processing",
   "nodes": [
     {
@@ -41,6 +42,10 @@
             {
               "name": "Idempotency-Key",
               "value": "={{$json.event_id}}"
+            },
+            {
+              "name": "X-Service-Token",
+              "value": "={{$env.MOBILITYOPS_CALLBACK_TOKEN}}"
             }
           ]
         },
@@ -113,7 +118,7 @@
   "settings": {
     "executionOrder": "v1"
   },
-  "active": false,
+  "active": true,
   "versionId": "11111111-1111-4111-8111-111111111111",
   "meta": {
     "templateCredsSetupCompleted": false