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>

tasks/orchestrate.py

@@ -380,7 +380,8 @@ def run_recipe(
     # Store in cache (content-addressed, auto-pins to IPFS)
     # Plan is just another node output - no special treatment needed
     cached, plan_ipfs_cid = cache_mgr.put(tmp_path, node_type="plan", move=True)
-    logger.info(f"Plan cached: hash={cached.cid}, ipfs={plan_ipfs_cid}")
+    plan_cache_id = plan_ipfs_cid or cached.cid  # Prefer IPFS CID
+    logger.info(f"Plan cached: cid={plan_cache_id}, ipfs={plan_ipfs_cid}")
 
     # Phase 4: Execute
     logger.info("Phase 4: Executing plan...")
@@ -392,7 +393,7 @@ def run_recipe(
         "run_id": run_id,
         "recipe": compiled.name,
         "plan_id": plan.plan_id,
-        "plan_cache_id": cached.cid,
+        "plan_cache_id": plan_cache_id,
         "plan_ipfs_cid": plan_ipfs_cid,
         "output_path": result.get("output_path"),
         "output_cache_id": result.get("output_cache_id"),