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/remix.py

@@ -3,6 +3,7 @@ cached original + rembg output and user-chosen filters/blends/opacity."""
 
 from __future__ import annotations
 
+import uuid
 from dataclasses import dataclass
 
 from . import pipeline
@@ -52,8 +53,8 @@ def create_remix_variant(job_id: str, choice: RemixChoice) -> dict:
         "variants": [],
     }
     variant_stem = pipeline.resolve_source_stem(manifest, paths)
-    manifest.setdefault("source_stem", variant_stem)
-    variant_id = pipeline.next_variant_id(manifest)
+    # Unique id avoids collisions when two remixes run in parallel.
+    variant_id = f"v{uuid.uuid4().hex[:8]}"
 
     entry = pipeline.compose_variant(
         paths,
@@ -67,6 +68,12 @@ def create_remix_variant(job_id: str, choice: RemixChoice) -> dict:
         fg_mode=choice.fg_blend,
         opacity=choice.opacity,
     )
-    manifest["variants"].append(entry)
-    pipeline.write_manifest(paths, manifest)
+    pipeline.append_manifest_variant(
+        paths,
+        entry,
+        original_file=paths.original.name,
+        rembg_file=paths.rembg.name,
+        source_stem=variant_stem,
+        created_at=manifest.get("created_at") if isinstance(manifest.get("created_at"), str) else None,
+    )
     return entry