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]

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[Priyanshu-byte-coder]

Good instinct on the serverless timeout, but firing pages 2..N as fully-parallel Promise.all fans out up to ~9 concurrent GitHub Search calls on a single user token. GitHub's own guidance is explicit: "make requests for a single user serially… do not make them concurrently," and Search has a low secondary rate limit — a burst like this risks tripping a secondary-rate-limit block, which is worse than the sequential latency it replaces. Please cap concurrency to a small pool (2-3 at a time) instead of unbounded parallel, then I'll take another look. Also: rate-limited pages now silently drop their items — that's fine as partial results, just confirm that's intended.