[Good First Issue] Add OpenAI GPT adapter for multi-provider support

[pixelstrade-dev]

Description

Currently CAIMS only supports Anthropic Claude as LLM provider. We need to add an OpenAI GPT adapter to enable multi-provider support.

What to do

1. Create a new file apps/web/lib/adapters/openai.ts following the pattern in apps/web/lib/adapters/anthropic.ts
2. 2. Implement the same chat() interface returning { content, inputTokens, outputTokens }
3. 3. Use the openai npm package
4. 4. Add OPENAI_API_KEY to .env.example
5. 5. Add a provider selection mechanism in the chat route

Skills needed

• TypeScript
• • Basic understanding of LLM APIs

References

• Existing adapter: apps/web/lib/adapters/anthropic.ts
• • OpenAI API docs: https://platform.openai.com/docs