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.

compose.yaml

@@ -33,6 +33,7 @@ services:
       RAGCORE_COLLECTION: ${RAGCORE_COLLECTION:-internal-procedures}
       RAGCORE_API_TOKEN: ${RAGCORE_API_TOKEN:-}
       N8N_WEBHOOK_URL: ${N8N_WEBHOOK_URL:-http://n8n:5678/webhook/mobilityops-return}
+      N8N_CALLBACK_TOKEN: ${MOBILITYOPS_CALLBACK_TOKEN:-replace-me-n8n-callback-token}
     ports:
       - "8128:8000"
     depends_on:
@@ -66,6 +67,8 @@ services:
       N8N_BASIC_AUTH_USER: ${N8N_BASIC_AUTH_USER:-admin}
       N8N_BASIC_AUTH_PASSWORD: ${N8N_BASIC_AUTH_PASSWORD:-change-me}
       N8N_SECURE_COOKIE: "false"
+      N8N_BLOCK_ENV_ACCESS_IN_NODE: "false"
+      MOBILITYOPS_CALLBACK_TOKEN: ${MOBILITYOPS_CALLBACK_TOKEN:-replace-me-n8n-callback-token}
     ports:
       - "5678:5678"
     volumes: