fix: sync DB sessions in Celery, letterbox YuNet, dedupe detections

- Rewrite all vision tasks to use sync psycopg2 sessions instead of
  asyncpg — fixes 'another operation in progress' and event loop errors
  when Celery forks workers sharing the async connection pool
- Letterbox-pad images to exactly 640x640 for YuNet face detector
  (was crashing on non-square thumbnails)
- Deduplicate object detections per label per photo — keep highest
  confidence only to avoid photo_tags PK violation on multiple
  detections of the same class
- Add all queues (-Q default,high,low,vision) to worker command

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

docker-compose.yml

@@ -62,7 +62,7 @@ services:
       context: ./backend
       dockerfile: Dockerfile
     container_name: mulita-worker
-    command: sh -c "python -m app.services.vision.bootstrap_models && celery -A app.tasks.celery worker --loglevel=${LOG_LEVEL:-info} --concurrency=${CELERYD_CONCURRENCY:-4}"
+    command: sh -c "python -m app.services.vision.bootstrap_models && celery -A app.tasks.celery worker --loglevel=${LOG_LEVEL:-info} --concurrency=${CELERYD_CONCURRENCY:-4} -Q default,high,low,vision"
     volumes:
       - ./mulita.yml:/app/config/mulita.yml:ro
       - ${PHOTO_DIRS:-./photos}:/photos:rw