feat(backup): write database dumps gzip-compressed

Dumps stream through gzip into <db>.backup.sql.gz; no uncompressed copy touches the disk. Restore reads gzip and plain dumps by their magic bytes, verifies a gzip dump to its end before anything is dropped, and still finds the uncompressed names of older generations. The manifest names the compression.

🎯 Fix: 92 %
💥 Break: 20 %
💸 Cost: –
⏱️ Time: –
🛡️ Security: ~100 %
🏆 Score: 74
🚦 Gate: ⛔ break 20 % > 5

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/unit/restore/test_cli_version_flag.py

@@ -27,7 +27,10 @@ class TestVersionFlagReachesEveryEngine(unittest.TestCase):
             *required,
             *extra,
         ]
-        with patch.object(cli, target) as restore:
+        with (
+            patch.object(cli, target) as restore,
+            patch.object(cli, "locate", side_effect=lambda path: path),
+        ):
             self.assertEqual(cli.main(argv), 0)
         return restore.call_args.kwargs