feat: Implement API Caching, Resilient Request Retries, and Error Backoff (#219)

[yashvi-3106]

Description

This PR implements caching for the /api/user/:username endpoint to reduce response times and unnecessary outbound queries to the external LeetCode API wrapper. It also implements robust request retries with exponential backoff and randomized jitter inside the daily synchronization script to safeguard against rate limits (HTTP 429) or transient server errors. Lastly, it ensures that downstream API failures are handled gracefully, logging the errors and returning a clean JSON error response (or falling back to expired cached data if available).

Linked Issue

Fixes #219

Changes Made

• scripts/fetch-user-info.js: Updated the live API fetch logic to propagate network/status errors instead of swallowing them internally.
• server.js: Added a 2-minute TTL in-memory cache system (using a JS Map with self-pruning). Modified the /api/user/:username endpoint to utilize the cache, serve stale cache entries as a fallback when the downstream API fails, and return a graceful JSON 502 Bad Gateway payload if no cache exists.
• scripts/sync-leaderboard.js: Configured a global axios response interceptor to handle rate limiting (429), server errors (5xx), and timeout errors, retrying up to 3 times using exponential backoff and jitter.

Type of Change

• Bug fix
• New feature
• UI/Visual update
• Documentation update
• Refactor

Testing

• Tested locally
• Tested on mobile viewport (if applicable)
• No console errors introduced

Checklist

• My code follows the project's coding style
• I have formatted my code locally by running npx prettier --write . before submitting
• I am submitting my PR from a dedicated feature/* branch, not the main branch
• I have performed a self-review of my code
• My changes generate no new warnings or errors
• I have updated documentation if required
• I have linked the relevant issue

Screenshots / Screen Recording

(None required - backend/script logic changes)

[github-actions]

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[yashvi-3106]

@jagdish-15 you can review it!

[jagdish-15]

/format

[github-actions]

All done! 💅 I've run Prettier and pushed the formatting fixes to this PR.

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[jagdish-15]

/format

[github-actions]

All done! 💅 I've run Prettier and pushed the formatting fixes to this PR.

Just a heads up: Since this commit was pushed by a bot, GitHub won't automatically re-run your CI checks. To trigger them, you can either:

• Push an empty commit (git commit --allow-empty -m "Trigger builds" and push)
• Close and immediately reopen this Pull Request.