fix(video-render): Veo Vertex AI → Gemini API (T10 follow-up)

박재오 발견: Veo는 Gemini API key 단일로 충분 (ai.google.dev).
Vertex AI의 GCP project + service account JSON + GCS bucket 셋업 불필요.

변경:
- providers/veo.py: generativelanguage.googleapis.com/v1beta endpoint
  + x-goog-api-key 헤더 + response.generateVideoResponse.generatedSamples[0].video.uri
- .env.example: GOOGLE_PROJECT_ID/LOCATION/GCS_BUCKET/SA_JSON 4 변수 → GEMINI_API_KEY 1개
- docker-compose: GCP 4 env + SA JSON volume mount 제거, GEMINI_API_KEY 추가
- requirements.txt: google-cloud-storage 제거 (requests만 사용)

박재오 측 영향: /etc/webai/gcp-sa.json 더미 파일 + GCP_SA_JSON_HOST_PATH env 무관.
GEMINI_API_KEY 1개만 발급하여 .env에 추가하면 됨.

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

services/video-render/.env.example

@@ -10,11 +10,8 @@ INTERNAL_API_KEY=__copy_from_nas_dotenv__
 # Sora 2 (OpenAI)
 OPENAI_API_KEY=__paste_openai_key__
 
-# Veo 3.1 (Google Vertex AI)
-GOOGLE_PROJECT_ID=__paste_gcp_project_id__
-GOOGLE_LOCATION=us-central1
-GOOGLE_GCS_BUCKET=__paste_gcs_bucket_name__
-GOOGLE_APPLICATION_CREDENTIALS=/app/keys/gcp-sa.json
+# Veo (Google Gemini API — ai.google.dev. Vertex AI 경로 아님, GCS bucket 불필요)
+GEMINI_API_KEY=__paste_gemini_key__
 
 # Kling (PiAPI gateway)
 PIAPI_API_KEY=__paste_piapi_key__