url-shortener submodule: track branch next (default dev).

Remote main removed; submodule docs and .gitmodules updated.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/README.md

@@ -15,9 +15,9 @@ git submodule update --init backend/url-shortener
 1. **URL shortener changes** — work inside the submodule only:
    ```bash
    cd backend/url-shortener
-   git checkout main && git pull
+   git checkout next && git pull
    # edit, test, commit, push to url-shortener remote
-   git push origin main
+   git push origin next
    ```
 2. **Pin new submodule SHA in android_cast** — after the submodule push:
    ```bash