Self-hosted ntfy that doesn't expose a token-mint endpoint can still
authenticate over HTTP Basic. Add Username + Password fields to
NtfyConfig; the channel sends 'Authorization: Basic …' when token is
empty and username is set. Token wins when both are configured.
Form-side: two new optional fields next to the access token, with
the same write-only placeholder treatment as smtp_password (blank
on edit means 'keep stored value'). Username is round-tripped on
edit; password is masked.
Fixes flagged in spec review of f0a323e: ntfy POSTs need explicit
Content-Type: text/plain (the spec calls for it; ntfy works without
but explicit beats inferred); trim trailing slashes from server URL
to avoid double-slash when operators paste 'https://ntfy.sh/'.
steve