The server side of talking to Discourse, all stdlib crypto:
VerifyWebhook checks X-Discourse-Event-Signature (sha256 HMAC hex,
constant-time, with or without the sha256= prefix) over the raw
body; ParseWebhook/WebhookHandler turn that into a drop-in
http.HandlerFunc that 403s mis-signed deliveries before dispatch.
VerifySSO decodes + verifies a Discourse Connect login redirect
(HMAC over the base64 string), BuildSSOResponse signs the identity
answer (nonce + email/external_id/username/...), and SSOLogin does
the whole dance in one call, returning the redirect URL. Secrets
load from DISCOURSE_WEBHOOK_SECRET / DISCOURSE_SSO_SECRET, are
trimmed, never logged and never echoed in errors.
Part of Redmine 507 (Discourse Go client): the webhook helper is
what the fleet receiver and 495 delivery loop verify pushes with.
