DEV: Fix mismatched column types (#240)

The primary key is usually a bigint column, but the foreign key columns
are usually of integer type. This can lead to issues when joining these
columns due to mismatched types and different value ranges.

db/migrate/20241009160105_alter_customer_id_to_bigint.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class AlterCustomerIdToBigint < ActiveRecord::Migration[7.1]
+  def up
+    change_column :discourse_subscriptions_subscriptions, :customer_id, :bigint
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end