feat(05-04): fix storage factory to dispatch nextcloud to NextcloudBackend

- Previously both 'nextcloud' and 'webdav' providers were dispatched to WebDAVBackend
- Now 'nextcloud' uses NextcloudBackend (has list_folder); 'webdav' uses WebDAVBackend
- Both share identical constructor signature (server_url, username, password)
- Removes type: ignore[import] concern on nextcloud_backend — module now exists

backend/storage/__init__.py

@@ -114,7 +114,14 @@ async def get_storage_backend_for_document(
     elif provider == "onedrive":
         from storage.onedrive_backend import OneDriveBackend  # type: ignore[import]
         return OneDriveBackend(credentials)
-    elif provider in ("nextcloud", "webdav"):
+    elif provider == "nextcloud":
+        from storage.nextcloud_backend import NextcloudBackend  # type: ignore[import]
+        return NextcloudBackend(
+            credentials["server_url"],
+            credentials["username"],
+            credentials["password"],
+        )
+    elif provider == "webdav":
         from storage.webdav_backend import WebDAVBackend  # type: ignore[import]
         return WebDAVBackend(
             credentials["server_url"],