[Bug]: Move MongoDB URI to .env File for Improved Configuration Management

[Pulkitxm]

Is there an existing issue for this?

• [X] I have searched the existing issues

What happened?

What happened? The MongoDB URI is currently hardcoded in the db.js file as follows:

const MONGODB_URI = "mongodb://localhost:27017/taskmanager";

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![image](https://github.com/user-attachments/assets/77848731-14f6-453b-ad3c-1fa2e088ae31)

### What browsers are you seeing the problem on?

Chrome

### Record

- [X] I agree to follow this project's Code of Conduct
- [X] I want to work on this issue
- [X] I'm willing to provide further clarification or assistance if needed.

[Pulkitxm]

@LakshmiSowmya04 I would like to work on this issue under hacktoberfest