Refactor backend and frontend cleanup paths

backend/storage/cloud_backend_factory.py

@@ -0,0 +1,33 @@
+"""Factory for user-scoped cloud storage backends."""
+
+
+def build_cloud_backend(provider: str, credentials: dict):
+    if provider == "google_drive":
+        from storage.google_drive_backend import GoogleDriveBackend  # lazy import
+
+        return GoogleDriveBackend(credentials)
+
+    if provider == "onedrive":
+        from storage.onedrive_backend import OneDriveBackend  # lazy import
+
+        return OneDriveBackend(credentials)
+
+    if provider == "nextcloud":
+        from storage.nextcloud_backend import NextcloudBackend  # lazy import
+
+        return NextcloudBackend(
+            credentials["server_url"],
+            credentials["username"],
+            credentials["password"],
+        )
+
+    if provider == "webdav":
+        from storage.webdav_backend import WebDAVBackend  # lazy import
+
+        return WebDAVBackend(
+            credentials["server_url"],
+            credentials["username"],
+            credentials["password"],
+        )
+
+    raise ValueError(f"Unknown provider: {provider}")

backend/storage/exceptions.py

@@ -6,12 +6,8 @@ class CloudConnectionError(Exception):
     """Raised when a cloud provider signals a non-retryable connection problem.
 
     Attributes:
-        reason: "token_expired"  — access token expired; API layer can refresh and retry.
+        reason: "token_expired" — access token expired.
                 "invalid_grant" — refresh token revoked; user must reconnect.
-
-    The backend never updates the DB. The API layer (_call_cloud_op in cloud.py)
-    catches this exception, performs the DB state transition, and decides whether
-    to retry or surface a 503 to the client (B2 design, D-05/D-06).
     """
 
     def __init__(self, msg: str = "", *, reason: str = "") -> None:

backend/storage/google_drive_backend.py

@@ -11,10 +11,8 @@ Design notes:
   - D-14: presigned_get_url and generate_presigned_put_url raise
     NotImplementedError. The API upload endpoint detects cloud backends and
     uses the direct put_object() path instead.
-  - B2 design: This backend is STATELESS. It raises CloudConnectionError but
-    does NOT update the DB or CloudConnection objects. DB state transitions
-    (e.g., REQUIRES_REAUTH) are handled by the _call_cloud_op() helper in
-    cloud.py (Plan 05-05), which has the DB session.
+  - This backend is stateless. It raises CloudConnectionError but does not
+    update the DB or CloudConnection objects.
   - Token key format stored in credentials dict:
       access_token   — current OAuth bearer token
       refresh_token  — long-lived refresh token

backend/storage/onedrive_backend.py

@@ -10,9 +10,8 @@ Design notes:
     already async and awaited directly.
   - CloudConnectionError is imported from google_drive_backend (shared type).
     This keeps the exception hierarchy unified across all cloud backends.
-  - B2 design: This backend is STATELESS. It raises CloudConnectionError but
-    does NOT update the DB or CloudConnection objects. DB state transitions
-    (e.g., REQUIRES_REAUTH) are handled by _call_cloud_op() in cloud.py.
+  - This backend is stateless. It raises CloudConnectionError but does not
+    update the DB or CloudConnection objects.
   - _ensure_valid_token() checks expiry before each API call and calls
     _refresh_token() if the token is within 60 seconds of expiry. If the
     refresh returns None (invalid_grant), CloudConnectionError is raised.