Send GM notification email asynchronously

Page response no longer waits on SMTP — status update persists and
returns immediately; email fires in a background goroutine.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/web/routes.go

@@ -167,9 +167,11 @@ func updatePlayerAttending(name, attending string, session data.Session) error {
 			if gmEmail := session.GMEmail(); gmEmail != "" {
 				subject, body := session.FormatStatusUpdate(name, newAttending, "the site")
 				e := email.NewEmail([]string{gmEmail}, []string{}, gmEmail, subject, body)
+				go func() {
 					if err := email.SendEmail(e); err != nil {
 						log.Default().Printf("ERROR sending GM notification: %s", err)
 					}
+				}()
 			}
 			return nil
 		}