fix: prevent MongoDB connection leak by reusing cached client connection

[ayuxsh009]

Description

This PR fixes a MongoDB connection leak in src/app/api/cohort/route.ts by replacing repeated client.connect() calls with a shared cached MongoDB client connection.

Previously, every incoming request attempted to establish a new MongoDB connection without properly closing it, which could eventually exhaust the MongoDB connection pool and degrade application performance.

Fixes #76

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Changes

• Fix
• Added src/lib/mongodb.ts singleton connection manager
• Replaced per-request client.connect() calls with shared clientPromise
• Added development-safe global caching for HMR hot reloads
• Optimized production connection reuse per serverless instance
• Updated API route logic in src/app/api/cohort/route.ts

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📷 Screenshots / Demo (if applicable)

N/A — backend connection lifecycle fix.

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Additional Notes

Problem

The previous implementation created a new MongoDB connection on every request:

await client.connect();

without ever closing or reusing the connection.

This could lead to:

• MongoDB connection pool exhaustion
• Increased memory/resource usage
• Performance degradation under load
• Connection timeout issues

Solution

This PR introduces a centralized MongoDB connection utility using a singleton clientPromise pattern:

• Ensures only one active connection per process/runtime
• Prevents duplicate connections during development hot reloads
• Improves scalability and resource efficiency

Files Changed

• src/lib/mongodb.ts (new)
• src/app/api/cohort/route.ts (modified)

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Checklist

• I have tested this code
• I have added necessary documentation
• I have linked relevant issue(s)
• I followed the code style of the project
• I reviewed my own code

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