refactor: add githubFetch helper to centralize GitHub API fetch logic (#959)

[mallya-m]

Summary

Closes #959

Creates src/lib/github-fetch.ts with typed helpers to eliminate the repeated ~10-line GitHub API fetch pattern across 8+ metric routes.

Type of Change

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

Changes Made

• Added githubFetch<T>() — typed REST fetch with Authorization header, Accept header, ok-check, and 403/429 rate limit handling
• Added githubGraphQL<T>() — typed GraphQL fetch helper
• Added GitHubRateLimitError and GitHubApiError typed error classes
• Re-exported GITHUB_API constant for convenience

How to Test

1. Import githubFetch in any metric route instead of raw fetch
2. Run npm run type-check — no errors
3. No behavior change — same error handling, same response types

Checklist

• Linked issue in summary
• No TypeScript errors
• Self-reviewed the diff
• No behavior change

[vercel]

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[github-actions]

GSSoC Label Checklist 🏷️

@Priyanshu-byte-coder — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus

[mallya-m]

@Priyanshu-byte-coder PR is ready — created src/lib/github-fetch.ts with typed githubFetch<T>() and githubGraphQL<T>() helpers plus typed error classes.

Could you please add:

• gssoc:approved
• level:intermediate
• type:feature type:refactor

Ready for merge!