Refactor backend architecture and update documentation

- Add CSRF protection middleware implementation
- Update API client with improved configuration
- Enhance documentation for backend development
- Add architecture documentation updates
- Reorganize and clean up task documentation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/middleware/__init__.py

@@ -0,0 +1,3 @@
+"""Application middleware."""
+
+from __future__ import annotations