Add Plaid migration constant alias (#1235)

Restore the original migration constant name as an alias so old references still resolve during production boot and migration loading. Add a regression test covering the legacy alias.

test/migrations/scope_plaid_item_uniqueness_migration_test.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+require "test_helper"
+require Rails.root.join("db/migrate/20260219200001_scope_plaid_item_uniqueness")
+
+class ScopePlaidItemUniquenessMigrationTest < ActiveSupport::TestCase
+  test "defines the legacy migration constant alias" do
+    assert_equal ScopePlaidItemUniqueness, ScopePlaidAccountUniquenessToItem
+  end
+end