fix(video-render): Veo durationSeconds str → int (T10 follow-up 2)

end-to-end 검증 2차: Gemini API는 durationSeconds를 number로 요구.
str("6") → 400 INVALID_ARGUMENT. int(params["duration"])로 전송.
(WebFetch 문서는 string으로 표기했으나 실제 API는 number.)

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

services/video-render/providers/veo.py

@@ -55,7 +55,7 @@ def run_veo_generation(task_id: str, params: dict) -> None:
         if params.get("number_of_videos"):
             body["parameters"]["numberOfVideos"] = int(params["number_of_videos"])
         if params.get("duration"):
-            body["parameters"]["durationSeconds"] = str(params["duration"])
+            body["parameters"]["durationSeconds"] = int(params["duration"])
         if params.get("resolution"):
             body["parameters"]["resolution"] = params["resolution"]
         if params.get("negative_prompt"):