Create OpenAI API-compatible HTTP interface for mellea

[psschwei]

Description:
Create an OpenAI API-compatible HTTP server that wraps mellea, allowing any OpenAI-compatible client to use mellea backends.

Key deliverables:

• mellea/integrations/openai_compat/server.py
• FastAPI/Starlette server implementing OpenAI chat completions endpoint
• /v1/chat/completions endpoint with streaming support
• Model listing endpoint
• Configuration for backend selection

Acceptance criteria:

• OpenAI Python SDK works against the server
• Streaming works correctly
• Tool calling works
• Can be deployed as standalone service

Notes:

• This enables "any framework that supports OpenAI" to use mellea
• Consider using existing libraries like litellm proxy as reference