Fix issue #52: Add a new speaker to the panel

[chuanli11]

This pull request fixes #52.

• Added Lambda API support end-to-end:
  • Introduced LAMBDA_API_KEY in .env.template and loaded it in config.py.
  • Implemented llm/lambda_client.py using the OpenAI-compatible client pointed at https://api.lambda.ai/v1 and set the model to deepseek-llama3.3-70b. The client provides generate_response with proper message formatting and retries.
• Integrated the new client into the app:
  • main.py registers "deepseek" as a panelist LLM, adds a user-facing name, and initializes the client with the Lambda API key.
  • TurnManager includes "deepseek" in panelist_ids so it participates in the panel discussion.
  • UI adds a color mapping for "deepseek".
• Tests updated to cover initialization and (optionally) real API integration, and to assert the new panelist is present.

These changes ensure the deepseek-llama3.3-70b model from the Lambda inference API is available and actively participates as a panelist in the discussion.

Automatic fix generated by OpenHands 🙌