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/services/vision/bootstrap_models.py

@@ -67,15 +67,38 @@ def bootstrap(models_dir: str | None = None):
 
     if missing:
         logger.warning(
-            "Missing %d model(s) that require manual export via export_models.py:",
+            "Missing %d model file(s); attempting automatic export:",
             len(missing),
         )
         for rel_path, desc in missing:
             logger.warning("  %s — %s", base / rel_path, desc)
-        logger.warning(
-            "Run: python -m app.services.vision.export_models --models-dir %s",
-            base,
-        )
+
+        try:
+            from app.services.vision import export_models
+
+            export_models.export_openclip(base)
+            export_models.export_yolov8n(base)
+        except Exception as e:
+            logger.error(
+                "Automatic export failed: %s. "
+                "Run `python -m app.services.vision.export_models "
+                "--models-dir %s` manually before starting the worker.",
+                e,
+                base,
+            )
+            return
+
+        # Re-check what's still missing after the export pass.
+        still_missing = [
+            (rel_path, desc)
+            for rel_path, desc in EXPORTS
+            if not (base / rel_path).exists()
+        ]
+        if still_missing:
+            for rel_path, desc in still_missing:
+                logger.error("  still missing: %s — %s", base / rel_path, desc)
+        else:
+            logger.info("All model files present in %s", base)
     else:
         logger.info("All model files present in %s", base)