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/execute_sexp.py

@@ -217,9 +217,9 @@ def execute_step_sexp(
 
         # Handle EFFECT nodes
         if node_type == "EFFECT":
-            effect_hash = config.get("hash")
+            effect_hash = config.get("cid") or config.get("hash")
             if not effect_hash:
-                raise ValueError("EFFECT step missing :hash")
+                raise ValueError("EFFECT step missing :cid")
 
             # Get input paths
             inputs = config.get("inputs", [])
@@ -277,7 +277,7 @@ def execute_step_sexp(
 
                 if filter_type == "EFFECT":
                     # Effect - for now identity-like, can be extended
-                    effect_hash = filter_config.get("hash") or filter_config.get("effect")
+                    effect_hash = filter_config.get("cid") or filter_config.get("hash") or filter_config.get("effect")
                     # TODO: resolve effect to actual FFmpeg filter
                     # For now, skip identity-like effects
                     pass