Fix registry lookups to use cid, remove dead legacy code

- Fix all registry lookups to use "cid" instead of "hash" key
  - app/routers/recipes.py: asset and effect resolution
  - tasks/execute_sexp.py: effect config lookups
  - server_legacy.py references (now deleted)
- Prefer IPFS CID over local hash in cache operations
  - cache_service.py: import_from_ipfs, upload_content
  - orchestrate.py: plan caching
  - legacy_tasks.py: node hash tracking

Remove ~7800 lines of dead code:
- server_legacy.py: replaced by modular app/ structure
- tasks/*_cid.py: unused refactoring only imported by server_legacy

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

legacy_tasks.py

@@ -359,7 +359,7 @@ def execute_dag(self, dag_json: str, run_id: str = None) -> dict:
                 node_type=cache_node_type,
                 node_id=node_id,
             )
-            node_hashes[node_id] = cached.cid
+            node_hashes[node_id] = ipfs_cid or cached.cid  # Prefer IPFS CID
             if ipfs_cid:
                 node_ipfs_cids[node_id] = ipfs_cid
             logger.info(f"Cached node {node_id}: {cached.cid[:16]}... -> {ipfs_cid or 'no IPFS'}")