fix(repos): paginate repo-explorer to fetch all repos beyond 100 limit (#2843)

[BCA-krishna]

Closes #2843

Problem

The repo-explorer endpoint called /user/repos?per_page=100 once,
returning at most 100 repositories. Users with more than 100 repos
saw an incomplete list on the dashboard, and metrics derived from
that list (PR counts, streak) were also incomplete.

Fix

• Replaced the single fetch() call in repo-explorer/route.ts with
  the existing fetchUserRepos() helper from src/lib/github.ts, which
  already paginates up to 10 pages (1000 repos maximum).
• Added missing created_at and language fields to the GitHubRepo
  interface so the type accurately reflects the GitHub API response.
• Updated test mock objects to include the new required fields.

Testing

• npm run type-check — 0 errors
• npm run lint — 0 errors

[github-actions]

GSSoC Label Checklist 🏷️

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

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

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

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