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_password_stdin.py

@@ -24,6 +24,7 @@ class TestRestorePasswordChannel(unittest.TestCase):
         stderr = io.StringIO()
         with (
             patch.object(cli, "restore_postgres_sql") as restore,
+            patch.object(cli, "locate", side_effect=lambda path: path),
             patch("sys.stdin", io.StringIO(stdin)),
             contextlib.redirect_stderr(stderr),
         ):