feat(deep-scan): trigger accepts 'deep'; scan_directory(mode) + enqueue backfill_phash

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/import_admin.py

@@ -15,14 +15,13 @@ import_admin_bp = Blueprint("import_admin", __name__, url_prefix="/api/import")
 async def trigger_scan():
     body = await request.get_json(silent=True) or {}
     mode = body.get("mode", "quick")
-    if mode != "quick":
-        # FC-2d will accept 'deep'. For now, reject anything else.
-        return jsonify({"error": f"mode {mode!r} not supported in FC-2a; use 'quick'"}), 400
+    if mode not in ("quick", "deep"):
+        return jsonify({"error": f"mode {mode!r} not supported; use 'quick' or 'deep'"}), 400
 
     # Enqueue the scan task in Celery. The task creates the batch itself.
     from ..tasks.scan import scan_directory
 
-    async_result = scan_directory.delay(triggered_by="manual")
+    async_result = scan_directory.delay(triggered_by="manual", mode=mode)
     return jsonify({"celery_task_id": async_result.id, "mode": mode}), 202