fix(repo): define all_tenants/0 as empty for non-multitenant schema

lib/mv/repo.ex

@@ -19,4 +19,12 @@ defmodule Mv.Repo do
   def min_pg_version do
     %Version{major: 17, minor: 2, patch: 0}
   end
+
+  # This app does not use schema-based multitenancy, so there are no tenant
+  # schemas to migrate. Returning [] keeps the AshPostgres callback total
+  # rather than raising the default "not defined" error.
+  @impl true
+  def all_tenants do
+    []
+  end
 end