feat: strengthen backup and mobile workflows

backend/backup/storage.py

@@ -0,0 +1,52 @@
+import shutil
+from pathlib import Path
+
+
+def contained_file(root: Path, storage_name: str) -> Path:
+    root = root.resolve()
+    candidate = (root / storage_name).resolve()
+    if candidate.parent != root:
+        raise ValueError("attachment path escapes storage root")
+    return candidate
+
+
+def quarantine_files(root: Path, storage_names: list[str], quarantine: Path) -> list[tuple[Path, Path]]:
+    moved = []
+    quarantine.mkdir(parents=True, exist_ok=True)
+    try:
+        for name in storage_names:
+            source = contained_file(root, name)
+            if source.exists():
+                target = quarantine / name
+                target.parent.mkdir(parents=True, exist_ok=True)
+                source.replace(target)
+                moved.append((source, target))
+        return moved
+    except Exception:
+        restore_quarantine(moved)
+        raise
+
+
+def restore_quarantine(moved: list[tuple[Path, Path]]) -> None:
+    for original, quarantined in reversed(moved):
+        if quarantined.exists():
+            original.parent.mkdir(parents=True, exist_ok=True)
+            quarantined.replace(original)
+
+
+def restore_quarantine_dir(root: Path, quarantine: Path) -> None:
+    if not quarantine.exists():
+        return
+    for quarantined in sorted(quarantine.rglob("*")):
+        if not quarantined.is_file():
+            continue
+        relative = quarantined.relative_to(quarantine)
+        original = contained_file(root, relative.as_posix())
+        original.parent.mkdir(parents=True, exist_ok=True)
+        quarantined.replace(original)
+    shutil.rmtree(quarantine)
+
+
+def remove_quarantine(path: Path) -> None:
+    if path.exists():
+        shutil.rmtree(path)