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.

backend/schemas/task.py

@@ -4,7 +4,6 @@ from datetime import date, datetime
 from enum import Enum
 
 from models.task import TaskType, TaskStatus, ReviewDecision, AttachmentType, NoteType
-from models.user import DepartmentRole
 
 
 class TaskBase(BaseModel):
@@ -14,6 +13,7 @@ class TaskBase(BaseModel):
     start_date: Optional[date] = None
     deadline: Optional[date] = None
     status: str = "not_started"  # Changed from TaskStatus enum to str to support custom statuses
+    department: Optional[str] = None  # Standard or project-custom department, independent of assignee
 
 
 class TaskCreate(TaskBase):
@@ -31,6 +31,7 @@ class TaskUpdate(BaseModel):
     start_date: Optional[date] = None
     deadline: Optional[date] = None
     status: Optional[str] = None  # Changed from TaskStatus enum to str to support custom statuses
+    department: Optional[str] = None  # Standard or project-custom department, independent of assignee
     assigned_user_id: Optional[int] = None
 
 
@@ -70,6 +71,7 @@ class TaskListResponse(BaseModel):
     name: str
     task_type: str  # Changed from TaskType enum to str to support custom task types
     status: str  # Changed from TaskStatus enum to str to support custom statuses
+    department: Optional[str] = None  # Standard or project-custom department, independent of assignee
     start_date: Optional[date] = None
     deadline: Optional[date] = None
     project_id: int