Say every forge event on the bus through a signed system webhook, and announce a head or merge at once (novox/hq issue 729)

The gitea module keeps one system webhook for every event family, made or
corrected at start with the admin account (POST /admin/hooks, forge 1.27.3)
and a secret it mints and keeps. Its receiver, at the docker bridge's
gateway, refuses an unsigned request before reading it, a body over 5 MB,
and a signature that does not verify; it answers a verified delivery at once,
drops one already heard by its delivery id, and publishes each as
forge.<kind>.<action> on the module's events (its own stream, ADR 0363,
follows issue 725). A pull request opened, pushed to or merged is announced
at once. The polls look every five minutes only while deliveries arrive, and
every 30 s again, saying so loudly, when one finds what no event said or ten
minutes pass without a delivery. The announced records keep both paths from
announcing twice.

modules/gitea/client.ts

@@ -914,6 +914,11 @@ export class GiteaAdmin {
     return { status: res.status, body };
   }
 
+  /** One admin-API call with a JSON body, answering the status and body (the system webhook, novox/hq issue 729). */
+  call(method: string, path: string, body?: unknown): Promise<AdminResponse> {
+    return this.request(path, { method, ...(body === undefined ? {} : { body: JSON.stringify(body) }) });
+  }
+
   /** Fail with the forge's own message when a status the caller did not expect comes back. */
   private static fail(path: string, res: AdminResponse): never {
     const detail = typeof res.body === "string" ? res.body : JSON.stringify(res.body);