spec: update api consistency

Align optimistic locking conflict payload, update websocket docs, and adjust tests.

backend/app/api/tasks/router.py

@@ -407,10 +407,13 @@ async def update_task(
         if task_data.version != task.version:
             raise HTTPException(
                 status_code=status.HTTP_409_CONFLICT,
-                detail=(
-                    f"Version conflict: current_version={task.version}, "
-                    f"provided_version={task_data.version}"
-                ),
+                detail={
+                    "error": "conflict",
+                    "message": "Task has been modified by another user. Please refresh and retry.",
+                    "current_version": task.version,
+                    "provided_version": task_data.version,
+                    "your_version": task_data.version,
+                },
             )
 
     # Capture old values for audit and triggers