feat(vereinfacht): add client, sync flash and SyncContact change

- Application: create SyncFlash ETS table on start
- Vereinfacht: Client, SyncFlash, sync_member, format_error, sync_members_without_contact
- SyncContact change on Member create_member and update_member
- Member: attribute vereinfacht_contact_id, internal action set_vereinfacht_contact_id

lib/mv/application.ex

@@ -7,6 +7,8 @@ defmodule Mv.Application do
 
   @impl true
   def start(_type, _args) do
+    Mv.Vereinfacht.SyncFlash.create_table!()
+
     children = [
       MvWeb.Telemetry,
       Mv.Repo,