Add signals

There should be some signals for events like "authentication failed" and "authentication successful".

This would make it easy for an application to log those events (or forward them to systems such as fail2ban).

Start by locating the Flask authentication flow and the points where authentication success and failure are handled. The issue names no files or tests, so first determine the appropriate signal entry points and project conventions. Done means applications can receive distinct signals for both events for logging or forwarding to systems such as fail2ban.

Written by the indexing model from the issue text.