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/main.py

@@ -1,15 +1,35 @@
 import uuid
+from contextlib import asynccontextmanager
 
 from fastapi import FastAPI, HTTPException, Request
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.responses import JSONResponse
 
-from app.api.routers import audit, bookings, dashboard, data_quality, demo, vehicles
+from app.api.routers import (
+    audit,
+    bookings,
+    dashboard,
+    data_quality,
+    demo,
+    integrations,
+    vehicles,
+    workflows,
+)
 from app.core.config import get_settings
 from app.core.errors import AppError, error_body
+from app.services.dispatcher import start_background_dispatcher, stop_background_dispatcher
 
 settings = get_settings()
-app = FastAPI(title="MobilityOps API", version="0.1.0")
+
+
+@asynccontextmanager
+async def lifespan(_app: FastAPI):
+    start_background_dispatcher()
+    yield
+    stop_background_dispatcher()
+
+
+app = FastAPI(title="MobilityOps API", version="0.1.0", lifespan=lifespan)
 
 app.add_middleware(
     CORSMiddleware,
@@ -62,3 +82,5 @@ app.include_router(vehicles.router)
 app.include_router(bookings.router)
 app.include_router(audit.router)
 app.include_router(data_quality.router)
+app.include_router(workflows.router)
+app.include_router(integrations.router)