feat(backup): stricter databases.csv semantics + atomic SQL dumps

- read databases.csv with stable types (dtype=str, keep_default_na=False)
- validate database field: require '*' or concrete name (no empty/NaN)
- support Postgres cluster dumps via '*' entries (pg_dumpall)
- write SQL dumps atomically to avoid partial/empty files
- early-skip fully ignored volumes before creating backup directories
- update seed CLI to enforce new contract and update by (instance,database)
- adjust tests: sql dir naming + add E2E coverage for early-skip and '*' seeding

tests/e2e/test_e2e_cli_contract_dump_only_sql.py

@@ -13,15 +13,6 @@ class TestE2ECLIContractDumpOnlySql(unittest.TestCase):
             f"Expected '--dump-only-sql' to appear in --help output. Output:\n{out}",
         )
 
-    def test_help_does_not_mention_old_flag(self) -> None:
-        cp = run(["baudolo", "--help"], capture=True, check=True)
-        out = (cp.stdout or "") + "\n" + (cp.stderr or "")
-        self.assertNotIn(
-            "--dump-only",
-            out,
-            f"Did not expect legacy '--dump-only' to appear in --help output. Output:\n{out}",
-        )
-
     def test_old_flag_is_rejected(self) -> None:
         cp = run(["baudolo", "--dump-only"], capture=True, check=False)
         self.assertEqual(