Application.start ran mark_all_offline unconditionally, which meant
every "mix run"/"mix ecto.migrate" invocation would flip all
connected hosts to offline. Gate the call on Phoenix.Endpoint.server?
so non-serving boots don't disturb live state.
Errors produced by Collectors.Host were keyword tuples {:tag, msg}, which
Jason cannot encode — metric push crashed the channel. Convert them to
plain maps with :tag and :message fields.
Reporter.handle_info/2 returned {:ok, socket}, which Slipstream rejects
(GenServer-style {:noreply, socket} is the only valid return for that
callback, unlike handle_connect/handle_join/handle_disconnect).
