ntfy channels with authentication send anonymously

The notification repository mapper never returns `ntfyAuthType` or `ntfyUsername`. Alerts are sent with what the mapper returns, so `NtfyProvider` sees no auth type and posts without an Authorization header, even for channels saved with token or basic auth. The same fields are also missing from GET responses.

Introduced by #3839 (my PR, the mapper was missed). Fix is two lines plus a repository test; PR coming right after this.

Locate the notification repository mapper and its repository test, then compare the fields returned for ntfy channels with the data NtfyProvider uses for authentication. Done means token and basic-auth channels preserve ntfyAuthType and ntfyUsername for alert handling and GET responses, with the repository test passing.

Written by the indexing model from the issue text.