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/requirements.txt

@@ -8,7 +8,6 @@ sqlalchemy[asyncio]==2.0.25
 aiosqlite==0.19.0           # SQLite escape hatch (docker-compose.sqlite.yml override)
 asyncpg==0.29.0             # async Postgres driver (default)
 psycopg2-binary==2.9.9      # sync Postgres driver, used by Alembic CLI
-pgvector==0.2.5             # pgvector SQLAlchemy types
 alembic==1.13.1
 
 # Redis and Celery
@@ -42,11 +41,6 @@ watchfiles==0.21.0
 # Vision pipeline (ONNX Runtime CPU inference)
 onnxruntime==1.18.1
 open-clip-torch==2.24.0     # tokenizer + export helper; inference via ONNX
-transformers>=4.37.0        # HuggingFace tokenizer for SigLIP models
-ultralytics==8.4.37          # YOLOv8n export helper; inference via ONNX
-rapidocr-onnxruntime==1.3.22
-scikit-learn==1.4.0          # DBSCAN for face clustering
-insightface>=0.7.3           # RetinaFace + ArcFace face detection/recognition
 numpy>=1.26.0,<2.0
 
 # Utilities