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>

frontend/src/services/api.ts

@@ -297,6 +297,37 @@ export interface WorkerStatus {
   scan_errors: string[]
 }
 
+export interface PipelineStage {
+  key: string
+  label: string
+  done: number
+  total: number
+  hint: string
+  /** True when the stage legitimately runs on a subset of photos — e.g.
+   *  GPS / tags / OCR / faces — so 100% coverage is never expected and
+   *  the UI should not frame "missing" as a problem. */
+  partial?: boolean
+  /** True when the stage doesn't have a done/total progress semantic
+   *  (e.g. face clusters, duplicate groups — those are output counts,
+   *  not ratios). The UI renders a plain count instead of a bar. */
+  standalone?: boolean
+}
+
+export interface PipelineStats {
+  total_photos: number
+  total_images: number
+  embedder_model: string
+  stages: PipelineStage[]
+}
+
+export interface ScanStatus {
+  is_scanning: boolean
+  current_folder: string | null
+  processed_files: number
+  total_files: number
+  errors: string[]
+}
+
 export const library = {
   scan: async () => {
     const response = await api.post('/library/scan')
@@ -344,6 +375,14 @@ export const library = {
       return response.data
     },
 
+    /** Per-stage ingestion progress — thumbnails, EXIF, GPS, phash,
+     *  embeddings, object tags, OCR, faces, face clusters, duplicate
+     *  groups. Drives the Pipeline Progress card in Settings. */
+    pipelineStats: async (): Promise<PipelineStats> => {
+      const response = await api.get('/library/maintenance/pipeline-stats')
+      return response.data
+    },
+
     /** Dry-run count of photo rows whose files are no longer on disk
      *  (under a mounted source root). */
     missingStats: async (): Promise<MissingStats> => {