[AI-7]: setup api route for chat streaming

[kevinkim-ogp]

TL;DR

This PR adds the foundation for REST API endpoints with proper authentication and rate limiting:

• Creates a new /api route structure with middleware for authentication and rate limiting
• Extracts client IP detection into a reusable getClientIp helper with comprehensive tests
• Adds authentication middleware that reuses existing GraphQL auth logic
• Implements rate limiting for API endpoints (10 requests per minute per user/IP)
• Adds extensive test coverage for all new middleware components
• Updates Vite config to proxy /api requests during development

Why make this change?

• Subsequent PR adds an /api/chat route for streaming the chat response from Pair for the AI Builder
• Why not GraphQL with SSE?
  • Requires installing more packages
  • Need to add subscriptions
  • More config changes that this current approach

How to test?

• All existing graphql APIs work as intended
• API calls to /api adhere to authentication and rate-limits requirements
  • Rate limit is 10/user/second (very conservative approach since there is only 1 route, and we don't want to spam the LLM)

[kevinkim-ogp]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• [AI-11] PLU-605: chat feedback #1352
• refactor: AI form modal #1311
• [AI-10] PLU-604: Preview steps and create Pipe #1310
• [AI-9] PLU-603: Chat interface #1309
• [AI-8] PLU-584: add api for chat streaming #1307
• [AI-7]: setup api route for chat streaming #1306 👈 (View in Graphite)
• [AI-6]: add ai related deps for frontend #1305
• [AI-5]: create pipe from generated steps #1304
• [AI-4] PLU-595: frontend to enter prompt and generate steps #1303
• [AI-3] PLU-583: (backend) mutation to generate AI steps #1302
• [AI-2]: Bump typescript and eslint deps #1301 : 1 other dependent PR (#1312 )
• [AI-1]: Install deps and bump zod #1300
• trunk/ai

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