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/e2e/test_e2e_postgres_cluster_restore.py

@@ -1,5 +1,7 @@
 import unittest
 
+from baudolo.generation import CLUSTER_SUFFIX, SQL_DIR
+
 from .helpers import (
     POSTGRES_DATA_DIR,
     POSTGRES_IMAGE,
@@ -85,8 +87,8 @@ class TestE2EPostgresClusterRestore(unittest.TestCase):
         cls.hash, cls.version = latest_version_dir(cls.backups_dir, cls.repo_name)
         cls.dump = (
             backup_path(cls.backups_dir, cls.repo_name, cls.version, cls.pg_volume)
-            / "sql"
-            / f"{cls.pg_container}.cluster.backup.sql"
+            / SQL_DIR
+            / f"{cls.pg_container}{CLUSTER_SUFFIX}"
         )
 
         run(cls._restore_argv(), stdin_text="pgpw")