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_mariadb_empty_drop.py

@@ -20,6 +20,7 @@ class TestMariadbEmptyDrop(unittest.TestCase):
             sql.flush()
             with (
                 patch.object(mariadb_mod, "docker_exec", side_effect=_capture),
+                patch.object(mariadb_mod, "docker_exec_stream"),
                 patch.object(mariadb_mod, "_pick_client", return_value="mariadb"),
             ):
                 mariadb_mod.restore_mariadb_sql(