Remove the no-op MAINTENANCE_MODE setting (closes #317)

MAINTENANCE_MODE only made the healthcheck report
maintenanceMode: true; no request was ever served differently. It is
removed from the configuration, the healthcheck JSON, the README and
the Settings page, together with the unused Server.MaintenanceMode
method and the healthcheck's Config dependency, which existed only to
read it. A leftover MAINTENANCE_MODE in the environment is not read,
so it is ignored rather than refused.

Model: opus-5-5

internal/server/server.go

@@ -159,12 +159,6 @@ func (s *Server) Run() {
 	s.serve()
 }
 
-// MaintenanceMode returns whether the server is in maintenance
-// mode.
-func (s *Server) MaintenanceMode() bool {
-	return s.params.Config.MaintenanceMode
-}
-
 // enableSentry initialises the Sentry SDK when error reporting is
 // configured, and reports the failure when it is configured and cannot
 // be initialised. A DSN that is not set is not a failure: reporting