clean up routine task references upon task deletion

[Abhishek2005-ard]

Description

When a task is deleted via the single task deletion endpoint (deleteTask controller), the backend removes the task from the database but fails to clean up corresponding references in the Routine model. This leaves orphaned taskId references inside user routines, leading to database integrity issues and potential API errors when retrieving or modifying routines.

This PR resolves the issue by executing a $pull operation on the Routine collection inside the deleteTask handler to clear references to the deleted task, matching the cleanup logic already used in the bulkDeleteTasks handler.

Changes Made

• Updated deleteTask in backend/controllers/taskController.js to run Routine.updateMany with a $pull action targeting the deleted taskId.

Verification & Testing

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