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_pipeline.py

@@ -13,6 +13,7 @@ from app import pipeline
     [
         ("photo.jpg", "photo"),
         ("../evil", "evil"),
+        ("a/b", "a_b"),
         ("", "photo"),
         ("  spaced  ", "spaced"),
         ("foo@bar", "foo_bar"),
@@ -78,3 +79,56 @@ def test_list_job_ids(data_dir: Path) -> None:
     paths = pipeline.job_paths("b")
     pipeline.write_status(paths, "processing")
     assert sorted(pipeline.list_job_ids()) == ["a", "b"]
+
+
+def test_parallel_manifest_merge_keeps_all_variants(data_dir: Path) -> None:
+    """Concurrent write_manifest callers must not clobber each other's variants."""
+    from concurrent.futures import ThreadPoolExecutor
+
+    paths = pipeline.job_paths("race")
+    pipeline.write_status(paths, "processing")
+    pipeline.write_manifest(
+        paths,
+        {
+            "source_stem": "race",
+            "original_file": "race_original.jpg",
+            "rembg_file": "race_rembg.png",
+            "created_at": pipeline.now_iso(),
+            "variants": [],
+        },
+    )
+
+    def write_one(i: int) -> None:
+        pipeline.write_manifest(
+            paths,
+            {
+                "variants": [{"id": f"v{i}", "file": f"race_v{i}.png"}],
+            },
+        )
+
+    with ThreadPoolExecutor(max_workers=8) as pool:
+        list(pool.map(write_one, range(20)))
+
+    meta = pipeline.read_meta("race")
+    assert meta is not None
+    ids = {v["id"] for v in meta["variants"]}
+    assert ids == {f"v{i}" for i in range(20)}
+    assert meta["original_file"] == "race_original.jpg"
+
+
+def test_filter_timeout_is_thread_local() -> None:
+    import threading
+
+    results: dict[str, int] = {}
+
+    def worker(name: str, value: int) -> None:
+        with pipeline.filter_timeout(value):
+            results[name] = pipeline._active_filter_timeout()
+
+    t1 = threading.Thread(target=worker, args=("a", 11))
+    t2 = threading.Thread(target=worker, args=("b", 22))
+    t1.start()
+    t2.start()
+    t1.join()
+    t2.join()
+    assert results == {"a": 11, "b": 22}