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

tests/e2e/test_e2e_dump_only_fallback_to_files.py

@@ -133,22 +133,28 @@ class TestE2EDumpOnlyFallbackToFiles(unittest.TestCase):
         )
 
     def test_files_backup_exists_due_to_fallback(self) -> None:
-        p = backup_path(
-            self.backups_dir,
-            self.repo_name,
-            self.version,
-            self.pg_volume,
-        ) / "files"
+        p = (
+            backup_path(
+                self.backups_dir,
+                self.repo_name,
+                self.version,
+                self.pg_volume,
+            )
+            / "files"
+        )
         self.assertTrue(p.is_dir(), f"Expected files backup dir at: {p}")
 
     def test_sql_dump_not_present(self) -> None:
         # There should be no sql dumps because databases.csv had no matching entry.
-        sql_dir = backup_path(
-            self.backups_dir,
-            self.repo_name,
-            self.version,
-            self.pg_volume,
-        ) / "sql"
+        sql_dir = (
+            backup_path(
+                self.backups_dir,
+                self.repo_name,
+                self.version,
+                self.pg_volume,
+            )
+            / "sql"
+        )
         # Could exist (dir created) in some edge cases, but should contain no *.sql dumps.
         if sql_dir.exists():
             dumps = list(sql_dir.glob("*.sql"))