Add migration for table storing HelpScout credentials (#4346)

priv/repo/migrations/20240708120453_create_help_scout_credentials.exs

@@ -0,0 +1,14 @@
+defmodule Plausible.Repo.Migrations.CreateHelpscoutCredentials do
+  use Plausible
+  use Ecto.Migration
+
+  def change do
+    if ee?() do
+      create table(:help_scout_credentials) do
+        add :access_token, :binary, null: false
+
+        timestamps()
+      end
+    end
+  end
+end