fix: add rate limiting for ai insights endpoint

[Sammmyyyyyyy]

Summary

Adds per-user rate limiting to the /api/ai-insights endpoint to prevent excessive Groq API usage and reduce abuse risk from repeated requests.

Closes #957

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup

Changes Made

• Added lightweight in-memory per-user rate limiting for /api/ai-insights
• Limited requests to 5 per hour per authenticated user
• Added HTTP 429 responses when rate limits are exceeded
• Added Retry-After response headers
• Improved protection against repeated concurrent AI insight requests

How to Test

Steps for the reviewer to verify this works:

1. Run the project locally using npm run dev
2. Authenticate and access the /api/ai-insights endpoint
3. Send repeated requests to the endpoint
4. Verify that after 5 requests within one hour, the endpoint returns HTTP 429
5. Verify that the response includes a Retry-After header

Screenshots (if UI change)

N/A

Checklist

• Linked issue in summary
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Self-reviewed the diff
• Added/updated tests if applicable

[vercel]

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[github-actions]

GSSoC Label Checklist 🏷️

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[github-actions]

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