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/apps/document_app.py

@@ -367,7 +367,8 @@ def change_parser():
             return get_data_error_result(retmsg="Not supported yet!")
 
         e = DocumentService.update_by_id(doc.id,
-                                         {"parser_id": req["parser_id"], "progress": 0, "progress_msg": "", "run": "0"})
+                                         {"parser_id": req["parser_id"], "progress": 0, "progress_msg": "",
+                                          "run": TaskStatus.UNSTART.value})
         if not e:
             return get_data_error_result(retmsg="Document not found!")
         if "parser_config" in req:

api/db/__init__.py

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