Add FREESOUND_API_KEY to Docker environment

BGM auto-download was failing because the API key was not passed to container.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docker-compose.yml

@@ -9,6 +9,7 @@ services:
     restart: unless-stopped
     environment:
       - OPENAI_API_KEY=${OPENAI_API_KEY}
+      - FREESOUND_API_KEY=${FREESOUND_API_KEY}
       - WHISPER_MODEL=${WHISPER_MODEL:-medium}
       - REDIS_URL=redis://redis:6379/0
     volumes: