fix: remove LiteLLM MASTER_KEY requirement

Removed LITELLM_MASTER_KEY as it requires a database for virtual key
management. Security is already provided by HTTP Basic Auth on the
public Traefik endpoint. Internal Open WebUI communication doesn't
need additional API key auth.

Security layers:
- Public access: HTTP Basic Auth via Traefik
- Internal LiteLLM: Network isolation (no auth needed)

🤖 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: ${AI_LITELLM_API_KEY}
+      OPENAI_API_KEYS: sk-internal
 
       # WebUI configuration
       WEBUI_NAME: ${AI_WEBUI_NAME:-Pivoine AI}
@@ -95,7 +95,6 @@ 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