Save (or clear) the notify URL and/or notify_email for this project.

Accepts notify_url (preferred) or ntfy_url (legacy) key for the ntfy/webhook channel, and notify_email for the email channel. ntfy entries are canonicalized to the topic path segment only and suffixed with -2/-3/… if another project in this DB already uses the same topic. Emails and non-ntfy webhooks pass through verbatim.

After saving a non-empty notify_url, fires a welcome notification so ntfy.sh topics are created on first publish (avoids 404 on first real alert).