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/vereinfacht/changes/sync_contact.ex

@@ -0,0 +1,54 @@
+defmodule Mv.Vereinfacht.Changes.SyncContact do
+  @moduledoc """
+  Syncs a member to Vereinfacht as a finance contact after create/update.
+
+  - If the member has no `vereinfacht_contact_id`, creates a contact via API and saves the ID.
+  - If the member already has an ID, updates the contact via API.
+  Runs in `after_transaction` so the member is persisted first. API failures are logged
+  but do not block the member operation. Requires Vereinfacht to be configured
+  (Mv.Config.vereinfacht_configured?/0).
+  """
+  use Ash.Resource.Change
+
+  require Logger
+
+  @impl true
+  def change(changeset, _opts, _context) do
+    if Mv.Config.vereinfacht_configured?() do
+      Ash.Changeset.after_transaction(changeset, &sync_after_transaction/2)
+    else
+      changeset
+    end
+  end
+
+  # Ash calls after_transaction with (changeset, result) only - 2 args.
+  defp sync_after_transaction(_changeset, {:ok, member}) do
+    case Mv.Vereinfacht.sync_member(member) do
+      :ok ->
+        Mv.Vereinfacht.SyncFlash.store(to_string(member.id), :ok, "Synced to Vereinfacht.")
+        {:ok, member}
+
+      {:ok, member_updated} ->
+        Mv.Vereinfacht.SyncFlash.store(
+          to_string(member_updated.id),
+          :ok,
+          "Synced to Vereinfacht."
+        )
+
+        {:ok, member_updated}
+
+      {:error, reason} ->
+        Logger.warning("Vereinfacht sync failed for member #{member.id}: #{inspect(reason)}")
+
+        Mv.Vereinfacht.SyncFlash.store(
+          to_string(member.id),
+          :warning,
+          Mv.Vereinfacht.format_error(reason)
+        )
+
+        {:ok, member}
+    end
+  end
+
+  defp sync_after_transaction(_changeset, error), do: error
+end