[BE/UI] Business rule enforcement — deletion blocking and task unassignment (Phase 11)

[CRamsan]

Phase 11 — Business Rule Enforcement

Implement business rule enforcement for deletion blocking and automatic task unassignment. This is split from #410 which covered the entire MVP Polish epic.

POL-10: Block Deletion of Properties/Units with Active Tasks

UI-level blocking:

• Delete button for a Unit must be disabled if the unit has any tasks with status Open or InProgress
• Disabled button shows tooltip: "Cannot delete unit with [N] active tasks. Complete or cancel them first."
• Delete button for a Property must be disabled if any unit in that property has active tasks

Backend-level enforcement:

• When a delete request is received for a unit or property with active tasks, return HTTP 409 Conflict:

  { "error": "ACTIVE_TASKS_EXIST", "message": "Cannot delete. [N] active task(s) must be resolved first.", "activeTaskCount": N }

• App shows dialog: "Cannot delete. Resolve all active tasks first."
• Applies to both soft and hard deletes

POL-11: Auto-Unassign Open Tasks on Staff Removal

When a Staff member is removed from the org:

1. Query all tasks: assignee_id = removedUserId AND status IN ('Open', 'InProgress')
2. Set assignee_id = NULL on all matching tasks
3. Create admin notification for each unassigned task

Must run within the same database transaction as the membership removal (no partial failure).

Unit test requirements:

• Remove staff with 3 open tasks: verify all 3 unassigned
• Remove staff with 0 open tasks: verify no side effects
• Remove staff with Completed/Cancelled tasks: verify those not modified

Dependencies

• [BE] TaskController + TaskService + TaskDatastore (Phase 0) #387 — Task API backend (for active task count query)
• [BE] Organization Membership API Contracts and Backend Services (Phase 0) #426 — Org Membership backend (removeMember side effect implementation point)
• [UI] Team Management and Pending Invites (Phase 10) #434 — Team Management screen (trigger point for staff removal in UI)

What type of task is this?

Development