feat(P5.7): task edit restrictions — block body edits in undergoing/completed/closed, enforce assignee-only edit in open+assigned

app/api/routers/tasks.py

@@ -181,9 +181,38 @@ def update_task(task_id: int, task_update: schemas.TaskUpdate, db: Session = Dep
     task = db.query(Task).filter(Task.id == task_id).first()
     if not task:
         raise HTTPException(status_code=404, detail="Task not found")
-    ensure_can_edit_task(db, current_user.id, task)
-    
+
+    # P5.7: status-based edit restrictions
+    current_status = task.status.value if hasattr(task.status, 'value') else task.status
     update_data = task_update.model_dump(exclude_unset=True)
+
+    # Fields that are always allowed regardless of status (non-body edits)
+    _always_allowed = {"status"}
+    body_fields = {k for k in update_data.keys() if k not in _always_allowed}
+
+    if body_fields:
+        # undergoing/completed/closed: body edits forbidden
+        if current_status in ("undergoing", "completed", "closed"):
+            raise HTTPException(
+                status_code=400,
+                detail=f"Cannot edit task body fields in '{current_status}' status. "
+                       f"Blocked fields: {sorted(body_fields)}",
+            )
+        # open + assignee set: only assignee or admin can edit body
+        if current_status == "open" and task.assignee_id is not None:
+            from app.api.rbac import is_global_admin, has_project_admin_role
+            is_admin = (
+                is_global_admin(db, current_user.id)
+                or has_project_admin_role(db, current_user.id, task.project_id)
+            )
+            if current_user.id != task.assignee_id and not is_admin:
+                raise HTTPException(
+                    status_code=403,
+                    detail="Only the current assignee or an admin can edit this task",
+                )
+
+    # Legacy general permission check (covers project membership etc.)
+    ensure_can_edit_task(db, current_user.id, task)
     if "status" in update_data:
         new_status = update_data["status"]
         old_status = task.status.value if hasattr(task.status, 'value') else task.status