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/cli.py

@@ -68,7 +68,7 @@ def parse_args() -> argparse.Namespace:
         action="store_true",
         help="Do not restart containers after backup",
     )
-    
+
     p.add_argument(
         "--dump-only-sql",
         action="store_true",