fix(performance): fetch GitHub commits concurrently to prevent serverless timeouts

[nyxsky404]

This PR refactors the GitHub Search API pagination in fetchContributionsForAccount. Previously, the route fetched up to 10 pages sequentially in a while loop, which could take up to 20 seconds for highly active users and frequently caused 504 Gateway Timeouts on Vercel's edge/serverless architecture. The new implementation fetches the first page to determine the total_count, and then fetches the remaining required pages (up to 10 max) concurrently using Promise.all(). This drastically reduces the total execution time while correctly handling secondary rate limit (HTTP 429/403) fallbacks. Fixes #3170.

[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

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

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