feat: optimize analytics via MongoDB aggregations and Redis caching

[PremSahith]

🛠️ Related Issue

Closes: #1297

📌 Description

This PR significantly optimizes the application's heavy data-processing endpoints by offloading complex operations from the Node.js event loop to MongoDB's Aggregation Engine. It also introduces a Redis caching layer for the most expensive, high-traffic endpoints (leaderboards) to prevent database strain and minimize latency at scale.

✨ Changes Made

• Database Indexing: Added compound indexes to the Progress, Analytics, and ExamResult models to allow MongoDB to efficiently $match and $sort data before aggregating it.
• Leaderboard Aggregation & Caching: Refactored progressController.js to use a $facet aggregation, allowing the system to compute the paginated leaderboard and total counts in a single DB query. Integrated a 10-minute Redis caching layer.
• Exam Analytics Optimization: Refactored examController.js's getMyResults to compute user attempts, best scores, and pass counts entirely in the database. Added a new /api/exam/leaderboard route with Redis caching to compute top scores globally or per-course.
• Analytics Aggregation Pipeline: Replaced expensive synchronous JavaScript loops in analyticsController.js (including buildWeeklyStats) with a powerful MongoDB $facet aggregation that computes weekly buckets, point totals, and event filtering natively in the DB layer.

📷 Screenshots

N/A - This is a purely backend performance and architectural optimization.

🧪 Type of Change

• Bug fix
• New feature
• UI/UX improvement
• Documentation update

✔️ Checklist

• My code follows project style guidelines
• I have tested my changes
• I have linked the related issue
• No unnecessary files are included

🚨 Notes for Contributors

• This PR introduces a Redis dependency. Ensure that a Redis server is running locally on port 6379, or configure the connection string via REDIS_URL in the .env file. (The application will gracefully fallback to direct database queries if Redis is unavailable).

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[PremSahith]

@JiyaBatra please review and merge this