Fix open-webui -> litellm auth: dummy key was rejected once LITELLM_MASTER_KEY is set

Open WebUI's OpenAI connection to LiteLLM was silently 401ing on every
request (OPENAI_API_KEY=dummy vs LiteLLM's real master key), so none of
the litellm-routed models (judge, tip-generator, kimi-agent, OpenRouter
free tier) ever appeared in the model picker -- only the direct Ollama
connection's models did. Key is now sourced from openwebui/.env
(gitignored), matching the langfuse key pattern already used elsewhere.

openwebui/docker-compose.yml

@@ -20,7 +20,7 @@ services:
       - ANTHROPIC_API_KEY=${ANTHROPIC_API_KEY}
       - OLLAMA_BASE_URL=http://host.docker.internal:11436
       - OPENAI_API_BASE_URL=http://host.docker.internal:4000/v1
-      - OPENAI_API_KEY=dummy
+      - OPENAI_API_KEY=${LITELLM_MASTER_KEY}
       # STT — Faster-Whisper large-v3-turbo
       - AUDIO_STT_ENGINE=openai
       - AUDIO_STT_OPENAI_API_BASE_URL=http://host.docker.internal:8880/v1