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/routers/admin.py

@@ -320,13 +320,8 @@ async def update_feature_flag(
 
 
 class BackfillVisionBody(BaseModel):
-    """POST body for triggering a vision backfill. ``task`` picks a
-    specific stage (``embed`` / ``ocr`` / ``detect`` / ``faces`` /
-    ``classify``); leaving it null runs every enabled stage. ``limit``
-    caps how many photos per stage are queued — useful for smoke-
-    testing a newly-enabled feature before committing a full run.
-    """
-    task: Optional[str] = None
+    """POST body for triggering a classifier backfill. ``limit`` caps how
+    many photos are queued."""
     limit: Optional[int] = None
 
 
@@ -335,61 +330,29 @@ async def trigger_ai_backfill(
     body: BackfillVisionBody,
     admin: User = Depends(require_admin),
 ):
-    """Queue a vision backfill pass. Identical code path as the automatic
-    post-scan backfill — just triggered manually from the UI."""
+    """Queue a classifier backfill pass."""
     if not is_enabled(FLAG_VISION_ENABLED):
         raise HTTPException(
             status_code=400,
             detail="Vision is currently disabled; enable it before running a backfill.",
         )
-    valid_tasks = {'embed', 'ocr', 'detect', 'faces', 'classify'}
-    if body.task is not None and body.task not in valid_tasks:
-        raise HTTPException(
-            status_code=400,
-            detail=f"task must be one of {sorted(valid_tasks)} or null",
-        )
     if body.limit is not None and body.limit <= 0:
         raise HTTPException(status_code=400, detail="limit must be positive")
 
-    # Import lazily so importing admin.py doesn't pull in the whole
-    # vision stack on startup (Celery task module loads numpy etc.).
     from app.tasks.vision import backfill_vision
 
-    result = backfill_vision.apply_async(
-        kwargs={'task': body.task, 'limit': body.limit}
-    )
+    result = backfill_vision.apply_async(kwargs={'limit': body.limit})
     logger.info(
         f"Admin '{admin.username}' queued vision backfill "
-        f"(task={body.task}, limit={body.limit}, celery_id={result.id})"
+        f"(limit={body.limit}, celery_id={result.id})"
     )
     return {
         "status": "queued",
         "task_id": result.id,
-        "task": body.task,
         "limit": body.limit,
     }
 
 
-@router.post("/ai/recluster-faces")
-async def trigger_face_recluster(admin: User = Depends(require_admin)):
-    """Kick off face recluster. Normally auto-fires after a scan via a
-    debounced scheduler; this endpoint is for admins who want to force
-    a fresh clustering pass (e.g. after tweaking ``cluster_eps`` in
-    the YAML config)."""
-    if not is_enabled(FLAG_VISION_ENABLED):
-        raise HTTPException(
-            status_code=400,
-            detail="Vision is currently disabled; enable it before reclustering.",
-        )
-    from app.tasks.vision import recluster_faces
-
-    result = recluster_faces.apply_async()
-    logger.info(
-        f"Admin '{admin.username}' queued face recluster (celery_id={result.id})"
-    )
-    return {"status": "queued", "task_id": result.id}
-
-
 @router.post("/ai/rescan")
 async def trigger_full_rescan(admin: User = Depends(require_admin)):
     """Dispatch the same scan_all_source_roots job the backend runs at