refactor: strip AI pipeline to binary photo/other classifier

Drops face recognition, OCR, object detection, and semantic embeddings.
The sole remaining vision task is a CLIP-based binary classifier
(photography vs other); photos in "other" get needs_review=true so
screenshots, documents, memes and scans can be triaged from a new
filter pill in the UI.

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

backend/app/tasks/celery.py

@@ -39,14 +39,9 @@ celery_app.conf.update(
     # Explicit routes for every task name. Wildcard patterns don't match
     # short names produced by @shared_task(name='...').
     task_routes={
-        # Vision queue — GPU/CPU-bound inference
-        'embed_photo': {'queue': 'vision'},
-        'ocr_photo': {'queue': 'vision'},
-        'detect_objects': {'queue': 'vision'},
-        'extract_faces': {'queue': 'vision'},
+        # Vision queue — CPU-bound binary classification
         'classify_content': {'queue': 'vision'},
         'vision_fanout': {'queue': 'vision'},
-        'recluster_faces': {'queue': 'vision'},
         # High-priority queue — thumbnails & duplicates
         'generate_thumbnails': {'queue': 'high'},
         'regenerate_all_thumbnails': {'queue': 'high'},