Add LiteLLM proxy service and config

[NoorChasib]

Introduce an OpenAI-compatible LiteLLM proxy: add llm_proxy service to .docker/compose.controller.yaml (port 8080, depends on llm_server, mounts config file, uses LITELLM_MASTER_KEY for client auth and LLM_API_KEY for backend auth). Add LLM_API_KEY and LITELLM_MASTER_KEY to .docker/.env-template (with generation hint). Add .docker/llm/litellm_config.yaml listing model_name -> backend mappings and general master_key/setting entries to route requests to various llm_server endpoints.

🎯 Summary

ZUBA-

🔰 Checklist

• I have read and agree with the following checklist

• I have performed a self-review of my code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation where required.
• I have tested my changes to the best of my ability.
• I have consulted with the team if introducing a new dependency.