revert: keep Bearer prefix for Vikunja Authorization header

The Bearer prefix is correct for Vikunja's API. The CI 401 error is
caused by an expired VIKUNJA_TOKEN secret, not the header format.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

tests/unit/test_api_clients.py

@@ -606,7 +606,7 @@ class TestVikunjaClient:
     def test_init_sets_headers(self) -> None:
         client = VikunjaClient("https://work.example.com", "tok")
         assert client._base_url == "https://work.example.com"
-        assert client._session.headers["Authorization"] == "tok"
+        assert client._session.headers["Authorization"] == "Bearer tok"
 
     def test_list_tasks(self) -> None:
         client = VikunjaClient("https://work.example.com", "tok")