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.

backend/app/core/config.py

@@ -15,6 +15,11 @@ class Settings(BaseSettings):
     ragcore_workspace: str = "mobilityops"
     ragcore_collection: str = "internal-procedures"
     n8n_webhook_url: str = "http://n8n:5678/webhook/mobilityops-return"
+    n8n_callback_token: str = "replace-me-n8n-callback-token"
+    n8n_dispatch_enabled: bool = True
+    n8n_dispatch_interval_seconds: float = 3.0
+    n8n_http_timeout_seconds: float = 5.0
+    n8n_max_attempts: int = 5
     app_secret: str = "replace-in-production"
     session_cookie_name: str = "mobilityops_session"
     session_ttl_seconds: int = 60 * 60 * 8