Use API authentication instead of token scraping

[joaopamaral]

Replace the HTML-scraping-based authentication with Superset's official security API endpoints.

Previously, UsernamePasswordAuth authenticated by:

1. Fetching /login/ and scraping the CSRF token from the HTML using BeautifulSoup
2. POSTing credentials to /login/ as form data

This was fragile and dependent on Superset's frontend HTML structure. The new approach uses the documented REST API:

1. POST /api/v1/security/login → obtains a JWT access token
2. GET /api/v1/security/csrf_token/ → obtains a CSRF token using the Bearer token

It was initially proposed in #319 (but I forgot to address the comments, sorry @Vitor-Avila)