Add filesystem fallback when artdag Cache lookup fails

The artdag Cache object doesn't persist state across process restarts,
so cache.get(node_id) returns None even when files exist on disk.

Now we check the filesystem directly at {cache_dir}/nodes/{node_id}/output.*
when the in-memory cache lookup fails but we have a valid node_id from
the Redis index.

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

cache_manager.py

@@ -457,6 +457,15 @@ class L1CacheManager:
                 logger.info(f"get_by_cid: Found via index: {path}")
                 return path
 
+            # artdag Cache doesn't know about entry - check filesystem directly
+            # Files are stored at {cache_dir}/nodes/{node_id}/output.*
+            nodes_dir = self.cache_dir / "nodes" / node_id
+            if nodes_dir.exists():
+                for f in nodes_dir.iterdir():
+                    if f.name.startswith("output."):
+                        logger.info(f"get_by_cid: Found on filesystem: {f}")
+                        return f
+
         # For uploads, node_id == cid, so try direct lookup
         # This works even if cache index hasn't been reloaded
         path = self.cache.get(cid)
@@ -465,6 +474,15 @@ class L1CacheManager:
             self._set_content_index(cid, cid)
             return path
 
+        # Check filesystem directly for cid as node_id
+        nodes_dir = self.cache_dir / "nodes" / cid
+        if nodes_dir.exists():
+            for f in nodes_dir.iterdir():
+                if f.name.startswith("output."):
+                    logger.info(f"get_by_cid: Found on filesystem (direct): {f}")
+                    self._set_content_index(cid, cid)
+                    return f
+
         # Scan cache entries (fallback for new structure)
         entry = self.cache.find_by_cid(cid)
         logger.info(f"get_by_cid: find_by_cid({cid[:16]}...) returned entry={entry}")