[GSSoC26] feat: Add MongoDB connection retry logic with exponential backoff

[ashrion]

Description

This PR adds robust MongoDB connection retry logic with exponential backoff.

Changes

• Implemented retry logic with exponential backoff (max 5 retries)
• Added jitter to prevent thundering herd problem
• Added connection event listeners (connected, error, disconnected)
• Added graceful shutdown handler for SIGINT
• Log detailed connection status and retry attempts

Motivation

• MongoDB connections can fail temporarily due to network issues
• Without retries, the app crashes or fails to start on transient failures
• Exponential backoff prevents overwhelming the database during recovery

Testing

• Start app with MongoDB down - verify retries and eventual success when DB comes up
• Verify connection events are logged correctly
• Verify graceful shutdown on SIGINT

Closes #453

GSSoC26 contribution