Store provenance on IPFS instead of local files

- Add add_json() to ipfs_client for storing JSON data
- Update render_effect task to store provenance on IPFS
- Update execute_dag task to store DAG provenance on IPFS
- Add provenance_cid field to RunStatus model
- Extract provenance_cid from task results

Provenance is now immutable and content-addressed, enabling:
- Cross-L2 verification
- Bitcoin timestamping for dispute resolution
- Complete audit trail on IPFS

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

server.py

@@ -151,6 +151,7 @@ class RunStatus(BaseModel):
     effect_url: Optional[str] = None  # URL to effect source code
     username: Optional[str] = None  # Owner of the run (ActivityPub actor ID)
     infrastructure: Optional[dict] = None  # Hardware/software used for rendering
+    provenance_cid: Optional[str] = None  # IPFS CID of provenance record
 
 
 # ============ Recipe Models ============
@@ -616,10 +617,12 @@ async def get_run(run_id: str):
                 if "output_hash" in result:
                     # New DAG result format
                     run.output_hash = result.get("output_hash")
+                    run.provenance_cid = result.get("provenance_cid")
                     output_path = Path(result.get("output_path", "")) if result.get("output_path") else None
                 else:
                     # Legacy render_effect format
                     run.output_hash = result.get("output", {}).get("content_hash")
+                    run.provenance_cid = result.get("provenance_cid")
                     output_path = Path(result.get("output", {}).get("local_path", ""))
 
                     # Extract effects info from provenance (legacy only)