Fix API routing and JWT auth endpoints

Delete backend/backend/urls.py

Fix API routing: add public months, summary, categories endpoints

Update urls.py

Fix Render deploy: move requirements.txt to backend

Delete backend/expenses/migrations directory

Delete backend/.venv directory

Add JWT auth, user-based expenses, Postgres config

Fix real-time expense state updates

Fix add expense modal behavior and improve UI update flow

Use Postgres instead of SQLite

Update frontend API base to production backend

Fix settings for Render deployment

Fix static files for Render deployment

Fix ALLOWED_HOSTS for Render