Publish alerts to a Sandbar topic and keep a Sandbar monitor for every resource with health checks

A Sandbar topic channel publishes alerts as ntfy-style messages (API key
optional). Under Server, Settings, Hammerhead can create, update and remove a
Sandbar monitor per enabled resource with health checks, linked by resource id
so monitors made by hand are left alone. The key is sealed at rest.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server.go

@@ -97,6 +97,7 @@ func (s *Server) Handler() http.Handler {
 	mux.HandleFunc("GET /api/admin/settings", s.withServerAdmin(s.getSettings))
 	mux.HandleFunc("PUT /api/admin/settings", s.withServerAdmin(s.putSettings))
 	mux.HandleFunc("POST /api/admin/settings/test-mail", s.withServerAdmin(s.testMail))
+	mux.HandleFunc("POST /api/admin/settings/sandbar-sync", s.withServerAdmin(s.syncSandbarNow))
 	mux.HandleFunc("GET /api/admin/update", s.withServerAdmin(s.checkUpdate))
 	mux.HandleFunc("POST /api/admin/update/apply", s.withServerAdmin(s.applyUpdate))
 	mux.HandleFunc("GET /api/admin/update/status", s.withServerAdmin(s.updateStatus))