fc5: backup + rollback services (pg_dump + tar.zst, manifest JSON, find-by-tag)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/services/migrators/rollback.py

@@ -0,0 +1,21 @@
+"""Restore from the most recent 'pre_migration'-tagged backup."""
+from __future__ import annotations
+
+from pathlib import Path
+
+from . import backup as backup_mod
+
+
+class NoBackupFoundError(Exception):
+    """Raised when rollback() is called with no pre_migration backup on disk."""
+
+
+def rollback_to_pre_migration(*, db_url: str, images_root: Path) -> dict:
+    manifest = backup_mod.find_latest_backup(images_root, tag="pre_migration")
+    if manifest is None:
+        raise NoBackupFoundError(
+            "no pre_migration-tagged backup found under <images_root>/_backups/"
+        )
+    return backup_mod.restore_backup(
+        manifest=manifest, db_url=db_url, images_root=images_root,
+    )