Fix unused variable in train_model endpoint

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vigilar/web/blueprints/pets.py

@@ -225,7 +225,6 @@ def delete_pet(pet_id: str):
 
 @pets_bp.route("/train", methods=["POST"])
 def train_model():
-    engine = _engine()
     pets_cfg = current_app.config.get("PETS_CONFIG")
     if not pets_cfg:
         return jsonify({"error": "Pet detection not configured"}), 400