fix(sf-b4): updated_at → snapshot_date for domrf_kn_objects (column doesn't exist)

Round-3 review caught second column mismatch. `domrf_kn_objects` has
`snapshot_date` (DATE NOT NULL), no `updated_at` — verified via
information_schema. CAST drop (snapshot_date is already DATE).

backend/app/api/v1/landing.py

@@ -77,7 +77,7 @@ def _query_stats(db: Session) -> dict[str, Any] | None:
         # KPI 5: дата последнего обновления (MAX snapshot_date из objective_lots + domrf)
         row_date_obj = db.execute(text("SELECT MAX(snapshot_date) FROM objective_lots")).scalar()
         row_date_domrf = db.execute(
-            text("SELECT MAX(CAST(updated_at AS date)) FROM domrf_kn_objects")
+            text("SELECT MAX(snapshot_date) FROM domrf_kn_objects")
         ).scalar()
 
         dates = [d for d in (row_date_obj, row_date_domrf) if d is not None]