A single WEBHOOK channel covers Gotify, Discord, Slack, Matrix, and Home
Assistant: the user supplies a public URL and an optional shared-secret
header, and the dispatcher POSTs a small plain-text JSON envelope. The
outbound goes through safeFetch with requirePublicHost so the SSRF floor
and the connect-time DNS-rebinding pin apply to the user-supplied host,
exactly as they do for ntfy. Responses classify 404/410/401/403 as hard
rejects (auto-disable) and 5xx/429/timeout as transient (backoff).
Settings card mirrors the ntfy card with a save + test-connection flow;
the header value is never returned by the GET and is preserved across a
save round-trip when left blank.
