fix(seed): handle empty databases.csv and add unit tests

- Gracefully handle empty databases.csv by creating header columns and emitting a warning
- Add _empty_df() helper for consistent DataFrame initialization
- Add unit tests for baudolo-seed including empty-file regression case
- Apply minor formatting fixes across backup and e2e test files

https://chatgpt.com/share/69628f0b-8744-800f-b08d-2633e05167da

src/baudolo/backup/app.py

@@ -72,6 +72,7 @@ def requires_stop(containers: list[str], images_no_stop_required: list[str]) ->
             return True
     return False
 
+
 def backup_mariadb_or_postgres(
     *,
     container: str,