Store DAG plan to IPFS and track plan_cid in run_cache

- Add plan_cid column to run_cache schema
- Store DAG JSON to IPFS during execute_dag task
- Return plan_cid in run status and list APIs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/run_service.py

@@ -132,6 +132,7 @@ class RunService:
                 "output_cid": cached.get("output_cid"),
                 "ipfs_cid": cached.get("ipfs_cid"),
                 "provenance_cid": cached.get("provenance_cid"),
+                "plan_cid": cached.get("plan_cid"),
                 "actor_id": cached.get("actor_id"),
                 "created_at": cached.get("created_at"),
                 "completed_at": cached.get("created_at"),