Description: Enable task assignment from one user to another with notification and state management.
Acceptance Criteria:
- Tasks can be assigned to other registered users
- Only task owner can assign tasks
- Assignment updates task state and assigned_to field
- Assignment validation ensures assignee exists
- Assignment history is tracked (who assigned, when)
- Assigned user receives notification of new assignment
Description: Enable task assignment from one user to another with notification and state management.
Acceptance Criteria: