feat: split celery workers, fix asyncpg-in-fork, add pipeline progress UI

Three overlapping fixes so the ingestion pipeline actually runs and the
user can see what it's doing:

Pipeline recovery
- app/database.py: use NullPool when MULITA_CELERY_WORKER=1 so each
  Celery task opens a fresh asyncpg connection on its own event loop.
  Fixes "another operation in progress" and "Future attached to a
  different loop" errors that were dropping ~every thumbnail +
  extract_metadata task on the floor.
- app/tasks/thumbs.py: initialize photo=None before the try and rollback
  on error so a transport failure in the initial SELECT doesn't raise
  UnboundLocalError in the except block and leak rows stuck in 'pending'.
- app/services/vision/bootstrap_models.py: on missing model files,
  invoke export_models automatically instead of just warning. First
  boot of a fresh install now self-heals.
- app/services/vision/export_models.py: shutil.move instead of
  Path.rename so the YOLO export survives the /app → /data/models
  cross-volume hop.
- requirements.txt: add ultralytics so export works in a stock image.

Worker topology
- docker-compose.yml: replace the single worker with worker-light
  (default/high/low queues, c=2, IO-bound) and worker-vision (vision
  queue, c=5, OMP_NUM_THREADS=1 to avoid oversubscription on 6 cores).
  Vision is pinned to ≤5 parallel inferences so ONNX doesn't each
  spawn an all-cores intra-op pool.
- .env / .env.example: CELERYD_CONCURRENCY replaced with
  CELERY_LIGHT_CONCURRENCY + CELERY_VISION_CONCURRENCY.
- Backfill queries in thumbs / scan / vision now ORDER BY taken_at
  DESC NULLS LAST so newest photos finish first — the library fills
  in top-down in the UI instead of arbitrary insertion order.

Settings visibility
- routers/library.py: new GET /maintenance/pipeline-stats returning
  done/total per stage (thumbnails, exif, gps, phash, embeddings,
  tags, ocr, faces, face clusters, duplicate groups). Worker-status
  now also reports the `vision` queue depth, which was missing.
- services/api.ts: PipelineStats / PipelineStage / ScanStatus types
  and the matching client call.
- components/dialogs/SettingsDialog.tsx:
  - new Pipeline Progress card with one progress bar per stage
  - inline scan banner (processed/total/current folder) inside the
    Library section while a scan is running
  - Tasks/min throughput computed by diffing worker processed counters
    between polls
  - Workers section calls out the vision queue and documents the
    CELERY_LIGHT/VISION_CONCURRENCY + docker compose up -d scale path

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/tasks/vision.py

@@ -446,12 +446,18 @@ def backfill_vision(task: str | None = None, limit: int | None = None):
     """Queue vision tasks for photos that haven't been processed yet.
     Uses a sync DB connection to avoid asyncpg conflicts in Celery."""
     model_name = settings.vision.embedder.name
+    # Newest-first ordering — matches regenerate_all_thumbnails so the
+    # whole ingestion pipeline sweeps the library top-down and the user
+    # sees recent photos fully-indexed long before the backlog drains.
+    # `taken_at` is the canonical capture timestamp (from EXIF, falls
+    # back to filesystem mtime in scan); `added_at` is the tie-breaker
+    # when taken_at is null.
     sql = """
         SELECT p.id FROM photos p
         LEFT JOIN embeddings e ON e.photo_id = p.id AND e.model = :model
         WHERE e.photo_id IS NULL
         AND p.processing_status = 'completed'
-        ORDER BY p.added_at DESC
+        ORDER BY p.taken_at DESC NULLS LAST, p.added_at DESC NULLS LAST
     """
     if limit:
         sql += f" LIMIT {limit}"