fix(video-render): Kling PiAPI → Native KlingAI (T11 follow-up)

박재오 발견: Kling 공식 API key 발급 (Access Key + Secret Key).
PiAPI gateway가 아닌 native api.klingai.com 사용.

변경:
- providers/kling.py: JWT 인증 (HS256, iss=access_key, exp=now+1800, nbf=now-5).
  POST /v1/videos/text2video → GET /v1/videos/{kind}/{task_id} 폴링.
  data.task_result.videos[0].url 다운로드.
  text2video / image2video 자동 분기.
- .env.example: PIAPI_API_KEY → KLING_ACCESS_KEY + KLING_SECRET_KEY
- docker-compose: 같은 env 교체
- requirements.txt: + PyJWT>=2.8.0

박재오 측: .env에 두 키 모두 입력.

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

services/docker-compose.yml

@@ -64,7 +64,8 @@ services:
       - INTERNAL_API_KEY=${INTERNAL_API_KEY:-}
       - OPENAI_API_KEY=${OPENAI_API_KEY:-}
       - GEMINI_API_KEY=${GEMINI_API_KEY:-}
-      - PIAPI_API_KEY=${PIAPI_API_KEY:-}
+      - KLING_ACCESS_KEY=${KLING_ACCESS_KEY:-}
+      - KLING_SECRET_KEY=${KLING_SECRET_KEY:-}
       - SEEDANCE_API_KEY=${SEEDANCE_API_KEY:-}
       - VIDEO_MEDIA_ROOT=${VIDEO_MEDIA_ROOT:-/mnt/nas/webpage/data/video}
       - VIDEO_MEDIA_URL_PREFIX=${VIDEO_MEDIA_URL_PREFIX:-/media/video}