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>

CLEANUP_PLAN.md

@@ -21,7 +21,7 @@ Phase 0 analysis (2026-07-18). Working branch: `cleanup/code-quality`.
 4. **Dead code**: `blend_layers()` unused
 5. **Duplication**: atomic JSON write ×3, `basicConfig` ×2, bool-env parsing ×2, compose env block ×2
 6. **Double meta read** on index page (`read_status` + `read_manifest`)
-7. **`compare-bg/`** — separate experiment, leave as-is (gitignored cache)
+7. ~~`compare-bg/`~~ — removed (was a standalone experiment)
 8. **`make_random.py`** — external reference only, document in ARCHITECTURE
 
 ## Risk matrix
@@ -63,6 +63,6 @@ Phase 0 analysis (2026-07-18). Working branch: `cleanup/code-quality`.
 - No `pipeline.py` module split in this pass
 - No meta file locking / concurrency fix
 - No dependency major version bumps
-- No changes to `compare-bg/` experiment
+- ~~No changes to `compare-bg/` experiment~~ (later removed entirely)
 - No deploy CI (manual `docker compose` on boka stays)
 - No behavior changes to gmic/rembg processing logic