Fix filter_config router import and mark Task 3 complete

backend/app/routers/filter_config.py

@@ -5,7 +5,7 @@ from typing import Annotated
 from fastapi import APIRouter, HTTPException, Path, Query, status
 
 from app.dependencies import AuthDep, Fail2BanConfigDirDep, Fail2BanSocketDep
-from app.services.config_file_service import ConfigWriteError
+from app.exceptions import ConfigWriteError
 from app.models.config import (
     AssignFilterRequest,
     FilterConfig,