Refactor: Use TaskStatus enum for task status handling (#646)

### What problem does this PR solve?

This commit changes the status 'not started' from being hard-coded to
being maintained by the TaskStatus enum. This enhancement ensures
consistency across the codebase and improves maintainability.

### Type of change

- [x] Refactoring

api/db/__init__.py

@@ -64,6 +64,7 @@ class ChatStyle(StrEnum):
 
 
 class TaskStatus(StrEnum):
+    UNSTART = "0"
     RUNNING = "1"
     CANCEL = "2"
     DONE = "3"