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_app_container_ships_client_tools.py

@@ -26,6 +26,7 @@ from .helpers import (
     backup_run,
     cleanup_docker,
     create_minimal_compose_dir,
+    dump_text,
     ensure_empty_dir,
     latest_version_dir,
     require_docker,
@@ -140,7 +141,7 @@ class TestE2EAppContainerShipsClientTools(unittest.TestCase):
     def test_the_engine_volume_was_dumped(self) -> None:
         dump = self.volume_dir(self.engine_volume) / SQL_DIR / f"shopdb{DUMP_SUFFIX}"
         self.assertTrue(dump.is_file(), f"expected a dump at {dump}")
-        self.assertIn(PAYLOAD, dump.read_text(encoding="utf-8"))
+        self.assertIn(PAYLOAD, dump_text(dump))
 
     def test_the_application_volume_produced_no_dump(self) -> None:
         """The collision this module exists for: the application container