Feat/spaced repetition 2279

[Vachhani-Tapan]

📝 Pull Request Description

Related Issue

Closes #2279

Summary

This PR implements the SuperMemo-2 (SM-2) algorithm to optimize user study sessions and flashcard retention. Instead of random selection, the backend now utilizes a RepetitionState model to dynamically schedule the optimal next review date based on a user's grade. A daily CRON job aggregates and queries for "due" cards, significantly increasing overall learning efficiency.

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Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature
• ♻️ Refactoring
• 📝 Documentation update
• 🎨 UI/UX improvement
• 🔥 Other(please describe) ______

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How Has This Been Tested?

• Deployed the Express backend and verified that the simulated daily CRON job loops reliably and successfully performs the mongoose query for $lte: today on the nextReviewDate.
• Unit tested the SM-2 utility to verify that a high grade (e.g., 4 or 5) correctly multiplies the interval based on the easinessFactor, while a low grade (e.g., 1 or 2) safely resets the repetitions state back to zero for immediate relearning.

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Screenshots (if applicable)

N/A - Backend algorithmic enhancement.

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Checklist

• My code follows the project's guidelines
• I have tested my changes
• I have updated documentation where necessary
• I have linked the related issue
• My changes do not introduce new warnings or errors

[github-actions]

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📥 Commits

Reviewing files that changed from the base of the PR and between 420348a and 5c509f0.

📒 Files selected for processing (5)

• backend/controllers/audioPipelineController.js
• backend/models/RepetitionState.js
• backend/server.js
• backend/utils/sm2.js
• frontend/src/hooks/useAudioPipeline.js

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