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>

tests/test_main.py

@@ -21,6 +21,16 @@ def test_validate_job_id_sanitizes_input() -> None:
     assert _validate_job_id("photo01") == "photo01"
 
 
+def test_validate_job_id_rejects_path_like() -> None:
+    with pytest.raises(HTTPException) as exc:
+        _validate_job_id("a/b")
+    assert exc.value.status_code == 400
+    with pytest.raises(HTTPException):
+        _validate_job_id("../evil")
+    with pytest.raises(HTTPException):
+        _validate_job_id("")
+
+
 def test_safe_job_file_rejects_wrong_prefix(data_dir: Path) -> None:
     stem = "photo01"
     paths = pipeline.job_paths(stem)