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/scan.py

@@ -422,7 +422,7 @@ async def _scan_all_source_roots_async():
     still hit Settings → Re-detect duplicates to force a fresh pass.
     """
     from app.tasks.thumbs import incremental_regroup_duplicates_task
-    from app.tasks.vision import backfill_vision, recluster_faces
+    from app.tasks.vision import backfill_vision
 
     async with AsyncSessionLocal() as session:
         result = await session.execute(
@@ -461,14 +461,6 @@ async def _scan_all_source_roots_async():
         except Exception as e:
             logger.warning(f"Could not queue post-scan vision backfill: {e}")
 
-        # 300s gives face extraction time to run before reclustering.
-        # Fires even if some faces are still in-flight — the task is
-        # idempotent and the user can re-trigger from Settings.
-        try:
-            recluster_faces.apply_async(countdown=300)
-        except Exception as e:
-            logger.warning(f"Could not queue post-scan face recluster: {e}")
-
         # Re-extract metadata for photos missing GPS coordinates.
         # Runs on every startup so photos scanned before the GPS fix
         # eventually get their coordinates populated.