feat: implement LRU cache eviction strategy in semanticContext middleware (#838)

[tanmayShinde-dev]

📝 Description

This PR enhances the in-memory caching mechanism in the semanticContext middleware by implementing a proper Least Recently Used (LRU) eviction strategy.

Improvements:

• Marks cache entries as recently used on every cache hit (delete → re-set pattern).
• Evicts the least recently used entry when CACHE_MAX_SIZE is exceeded.
• Retains existing TTL expiration logic.
• Ensures predictable memory usage and improved cache efficiency.

This change affects the Node.js API (backend middleware layer only).
It does not impact the Vanilla JS frontend or PWA service workers.

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🔗 Related Issue

Fixes #838

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🛠️ Type of Change

• ✨ New Feature (Backend performance improvement via LRU caching)
• 🐞 Bug Fix
• 📝 Documentation
• 🔐 Security/Backend
• 📱 PWA/Offline

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🧪 Testing & Validation

• Server starts successfully without runtime errors.
• Verified cache hit reordering logic works as expected.
• Confirmed least recently used entries are evicted when size limit is exceeded.
• Tested in full-stack mode with MongoDB (not required for cache logic validation).
• Ran npm run dev and confirmed no crashes or syntax errors.

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🚩 Checklist:

• Code follows the existing folder structure (middleware/semanticContext.js).
• No new environment variables were introduced.
• No sensitive .env secrets or private keys were committed.
• PR targets the main branch.

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📸 Visuals (if applicable)

Not applicable — backend-only improvement.

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[github-actions]

🎉 Thanks for the PR, @tanmayShinde-dev!

We really appreciate you taking the time to contribute to ExpenseFlow! 💙

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🔗 https://github.com/Renu-code123/ExpenseFlow

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✅ PR Checklist

Before we review, please ensure:

• Your code follows the project's coding standards
• All file changes are accurate and intentional
• You've tested your changes locally
• Any review comments have been addressed

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We truly appreciate your interest in contributing to this project.

• Please make sure your code follows the project structure
• Add clear commit messages and comments where necessary
• Ensure your changes do not break existing functionality

We'll review your PR as soon as possible. Keep up the great work! ✨

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Project	Deployment	Actions	Updated (UTC)
expenseflow	Ready	Preview, Comment	Mar 1, 2026 3:55pm