Add due_date field to Task model

[Mubashir78]

Add field (nullable Date) to the Task model, schemas, and related CRUD operations.

[Navashub]

Hey @Mubashir78 , thanks for picking up an issue and taking the time to contribute!
Before we review the code, there's one thing to fix first - your PR is coming from your main branch directly, instead of a dedicated feature branch. Per our CONTRIBUTING.md, all contributions should follow this branch naming convention:

git checkout -b feature/add-due-date
Here's how to fix it without losing your work:

On your fork, create a new branch from your current main
git checkout -b feature/add-due-date

Push it to your fork

git push origin feature/add-due-date
Then open a new PR from feature/add-due-date → Navashub:main and close this one.
This is important because working directly on main makes it harder to manage changes if revisions are needed, and it's a habit that will serve you well in any real-world project. The fix takes less than 2 minutes - looking forward to reviewing the actual code once the new PR is up!

[Mubashir78]

Closed in favor of #21 (now using proper feature branch feature/add-due-date per CONTRIBUTING.md)