fix: lock meta RMW and harden concurrent remix paths

Prevent worker/web clobbering of meta variants via flock and merge-by-id,
make filter timeouts thread-local, harden job-id/stem sanitization, migrate
TemplateResponse API, and remove the compare-bg experiment.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/io_utils.py

@@ -2,7 +2,11 @@
 
 from __future__ import annotations
 
+import fcntl
 import json
+import os
+from collections.abc import Iterator
+from contextlib import contextmanager
 from pathlib import Path
 from typing import Any
 
@@ -23,3 +27,16 @@ def write_json_atomic(path: Path, data: Any) -> None:
     tmp = path.with_suffix(path.suffix + ".tmp")
     tmp.write_text(json.dumps(data, indent=2, ensure_ascii=False), encoding="utf-8")
     tmp.replace(path)
+
+
+@contextmanager
+def file_lock(lock_path: Path) -> Iterator[None]:
+    """Exclusive advisory lock via ``fcntl.flock`` (cross-process)."""
+    lock_path.parent.mkdir(parents=True, exist_ok=True)
+    fd = os.open(lock_path, os.O_CREAT | os.O_RDWR, 0o644)
+    try:
+        fcntl.flock(fd, fcntl.LOCK_EX)
+        yield
+    finally:
+        fcntl.flock(fd, fcntl.LOCK_UN)
+        os.close(fd)