refactor(api): extract cache-control header builder into utility function

[lokeshkumar69]

Description

Closes #321

Extract inline Cache-Control header strings from app/api/streak/route.ts and app/api/github/route.ts into a shared utility function buildCacheControlHeader in utils/cacheControl.ts.

Pillar

♻️ Code Quality / Refactor

Checklist

• I have read CONTRIBUTING.md
• My code follows the project's code style
• I have added unit tests for all branches
• All existing tests pass (2543 tests ✅)
• No breaking changes introduced

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