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/seed_loader.py

@@ -235,16 +235,37 @@ def load_seed(db: Session) -> SeedResult:
     db.execute(insert(DataQualityIssue), dq_rows)
     counts["data_quality_issues"] = len(dq_rows)
 
+    vehicle_ref_by_booking_ref = {
+        row["public_ref"]: row["vehicle_ref"] for row in _read_csv("bookings.csv")
+    }
+
     outbox_rows = []
     for row in _read_csv("workflow_runs.csv"):
         booking_id = booking_id_by_ref.get(row["aggregate_ref"])
+        # Build the same schema-complete envelope the live return workflow (M2) produces,
+        # so a seeded/historical event is redeliverable (e.g. via manual retry) without the
+        # dispatcher crashing on a missing key. See PROJECT_STATE.md M4 notes.
         outbox_rows.append(
             {
                 "event_id": uuid.UUID(row["event_id"]),
                 "event_type": row["event_type"],
                 "aggregate_type": "booking",
                 "aggregate_id": booking_id or uuid.uuid4(),
-                "payload_json": {"aggregate_ref": row["aggregate_ref"]},
+                "payload_json": {
+                    "correlation_id": str(uuid.uuid4()),
+                    "aggregate": {
+                        "type": "booking",
+                        "id": str(booking_id or uuid.uuid4()),
+                        "public_ref": row["aggregate_ref"],
+                    },
+                    "data": {
+                        "vehicle_ref": vehicle_ref_by_booking_ref.get(row["aggregate_ref"], ""),
+                        "inspection_ref": "",
+                        "resulting_vehicle_status": "cleaning",
+                        "attention_reasons": [],
+                    },
+                    "aggregate_ref": row["aggregate_ref"],
+                },
                 "occurred_at": _parse_dt(row["occurred_at"]),
                 "delivery_status": row["status"],
                 "attempts": int(row["attempts"]),