feat: secure LiteLLM API key with environment variable

- Added AI_LITELLM_API_KEY environment variable to .env
- Configured LiteLLM MASTER_KEY for authentication
- Updated Open WebUI to use secure API key from environment
- Generated secure 64-character hex key: sk-77b42236...

This replaces the insecure hardcoded sk-1234 key with proper
secret management via environment variables.

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

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

ai/compose.yaml

@@ -35,7 +35,7 @@ services:
 
       # OpenAI API configuration (pointing to LiteLLM proxy)
       OPENAI_API_BASE_URLS: http://litellm:4000
-      OPENAI_API_KEYS: sk-1234
+      OPENAI_API_KEYS: ${AI_LITELLM_API_KEY}
 
       # WebUI configuration
       WEBUI_NAME: ${AI_WEBUI_NAME:-Pivoine AI}
@@ -95,6 +95,7 @@ services:
     environment:
       TZ: ${TIMEZONE:-Europe/Berlin}
       ANTHROPIC_API_KEY: ${ANTHROPIC_API_KEY}
+      LITELLM_MASTER_KEY: ${AI_LITELLM_API_KEY}
       DATABASE_URL: null
     volumes:
       - ./litellm-config.yaml:/app/litellm-config.yaml:ro