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/task.py

@@ -56,6 +56,7 @@ class Task(Base):
     name = Column(String, nullable=False, index=True)
     description = Column(Text)
     status = Column(String, nullable=False, default="not_started")  # Changed from Enum to String to support custom statuses
+    department = Column(String, nullable=True)  # Standard or project-custom department, independent of assignee
     start_date = Column(Date)
     deadline = Column(Date)
     created_at = Column(DateTime(timezone=True), server_default=func.now())