refactor(backend): Implement unified Message collection to fix 16MB limit (#25)

[abhinavkdeval08-design]

Description

This PR addresses issue #25 by refactoring the chat storage schema.
The previous implementation stored messages as embedded arrays within thread/chat documents, which could lead to MongoDB's 16MB document size limit bottleneck over time...

Changes Made:

• Created a new, unified Message.js model to store all DMs and Server messages independently.
• Updated controllers to query this new collection.
• Introduced Cursor Pagination limit to fetch 50 messages at a time instead of loading the entire array.

Fixes #25

---

Hi @0rigin-c0de, I have implemented the schema refactor as discussed. Please review the code whenever you have time, and kindly add the gssoc:approved and level:advanced type:feature type:performance labels to this PR so the GSSoC tracker can catch it. Thank you!

[vercel]

@abhinavkdeval08-design is attempting to deploy a commit to the Sunil Kumar's projects Team on Vercel.

A member of the Team first needs to authorize it.

[abhinavkdeval08-design]

Hi @0rigin-c0de,

I’ve submitted the PR for the schema refactor as discussed. This implementation moves messages out of embedded arrays into a unified collection, which permanently resolves the MongoDB 16MB document size bottleneck (#25).

Key highlights of this PR:

Scalability: Messages are now stored independently, preventing unbounded array growth.

Performance: Implemented cursor-based pagination (50 messages per fetch), which significantly reduces the initial payload size and improves load times.

Cleanup: The code has been refactored for better query maintainability.

Since this is an Advanced level task under GSSoC'26, could you please review the changes when you have a moment? Also, if the implementation aligns with your expectations, kindly add the relevant labels (gssoc:approved, level:advanced, type:feature, type:performance) so it can be tracked correctly.