Add per-project Department management

Departments are now a customizable per-project list (standard + custom),
matching the existing custom task type/status pattern, instead of a fixed
7-value enum. They're usable directly on tasks (new field, independent of
assignee) and continue to drive team member department roles.

frontend/src/services/task.ts

@@ -11,6 +11,7 @@ export interface Task {
   description?: string
   task_type: string
   status: TaskStatus
+  department?: string
   start_date?: string
   deadline?: string
   project_id: number
@@ -34,6 +35,7 @@ export interface TaskListItem {
   name: string
   task_type: string
   status: TaskStatus
+  department?: string
   start_date?: string
   deadline?: string
   project_id: number