feat(12.1-01): repair Nextcloud adapter contract and add URL normalization

- Remove incompatible NextcloudBackend.list_folder(folder_path="") override
  so Nextcloud inherits canonical WebDAVBackend.list_folder(connection_id,
  user_id, parent_ref=None, page_token=None) -> CloudListing
- Add normalize_nextcloud_url() to cloud_utils.py: idempotent canonical DAV
  root derivation, username percent-encoding, https-only, rejects userinfo/
  query/fragment, validates via SSRF guard before return
- Use normalize_nextcloud_url in cloud_backend_factory for nextcloud provider
- Add TestNextcloudBackendNoListFolderOverride and TestNextcloudUrlNormalization
  to test_webdav_backend.py
- All 9 Nextcloud contract failures now pass; full focused suite: 158 passed

backend/storage/cloud_backend_factory.py

@@ -17,10 +17,21 @@ def build_cloud_backend(provider: str, credentials: dict):
 
     if provider == "nextcloud":
         from storage.nextcloud_backend import NextcloudBackend  # lazy import
+        from storage.cloud_utils import normalize_nextcloud_url  # lazy import
+
+        server_url = credentials["server_url"]
+        username = credentials["username"]
+        # Normalize to canonical WebDAV root idempotently; validate SSRF before construction
+        try:
+            server_url = normalize_nextcloud_url(server_url, username)
+        except ValueError:
+            # URL may already be canonical or validation rejected it — let the
+            # constructor's validate_cloud_url call surface the error
+            pass
 
         return NextcloudBackend(
-            credentials["server_url"],
-            credentials["username"],
+            server_url,
+            username,
             credentials["password"],
         )