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/models/__init__.py

@@ -1,5 +1,5 @@
 # Models package
-from .user import User, UserRole, DepartmentRole
+from .user import User, UserRole
 from .project import Project, ProjectMember, ProjectStatus
 from .episode import Episode, EpisodeStatus
 from .asset import Asset, AssetCategory, AssetStatus
@@ -17,7 +17,7 @@ from .role import Role, Permission, role_permissions, user_roles
 
 __all__ = [
     # User models
-    "User", "UserRole", "DepartmentRole",
+    "User", "UserRole",
     # Project models
     "Project", "ProjectMember", "ProjectStatus",
     # Episode models