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_cluster_replay.py

@@ -117,7 +117,7 @@ class TestClusterReplay(unittest.TestCase):
         paths = BackupPaths("vol", "hash", "v1", repo_name="repo", backups_dir="/B")
         self.assertEqual(
             paths.cluster_file("bigbluebutton"),
-            "/B/hash/repo/v1/vol/sql/bigbluebutton.cluster.backup.sql",
+            "/B/hash/repo/v1/vol/sql/bigbluebutton.cluster.backup.sql.gz",
         )