fix: resolve timeframe data leak and unawaited writes

[Yashaswini-K-P]

Description

This PR refactors the student history file update system inside sync-leaderboard.js to eliminate Event Loop starvation and prevent operating system resource exhaustion.

Previously, user files were processed sequentially using blocking, synchronous operations (like fs.readFileSync and fs.writeFileSync) inside a heavy loop. As the user registry grew, this completely blocked the Node.js single thread (The Freeze). Transitioning these operations to uncontrolled parallel execution (e.g., a naive Promise.all()) would attempt to open hundreds of files simultaneously, immediately crashing the process with an EMFILE error due to operating system file descriptor limits (The Crash).

This change introduces controlled, asynchronous concurrency limits (chunking) paired with non-blocking Promise-based file system APIs to achieve high performance without compromising server responsiveness or system limits.

Linked Issue

Fixes #244

Changes Made

• Asynchronous Chunking (DISK_CONCURRENCY_LIMIT): Replaced synchronous file access with asynchronous operations executed in bounded concurrent batches of 20 at a time.
• Non-blocking Disk I/O: Migrated from synchronous fs methods to fsPromises and atomicWrite inside the user-data synchronization loop, freeing up the Node.js Event Loop to handle concurrent tasks.
• EMFILE Crash Prevention: Capped concurrent active file handles to safely remain far below system-level open file descriptor limits.

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

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[jagdish-15]

/format

[github-actions]

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

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