clean up routine task references upon task deletion

[Abhishek2005-ard]

Description

When a single task is deleted via the deleteTask controller, the backend deletes the task from the database but leaves its ID referenced inside any Routine models that used it. This leads to orphaned task references inside routines, resulting in database integrity errors.

This PR updates the deleteTask handler to pull the deleted task's ID from the items array of the Routine collection, ensuring referential integrity (matching the cleanup behavior of bulkDeleteTasks).

Changes Made

• Added a Routine.updateMany with a $pull operation in backend/controllers/taskController.js to remove references to the deleted task ID.

Verification & Testing

• Ran backend linter: npm run lint (Passed)
• Ran backend unit tests: npm run test (Passed)