fix: change postal_code column from integer to string (#1585)

* fix: change postal_code column from integer to string

Allows non-numeric postal codes such as UK format (e.g. SW1A 2AA).
The integer column was silently dropping any alphanumeric input.

The migration is marked irreversible — once alphanumeric postal codes
exist, they cannot be safely cast back to integer.

* fix: update schema.rb, quote fixture postal_code, and add alphanumeric test

* fix: use conventional migration timestamp

test/models/address_test.rb

@@ -39,6 +39,12 @@ class AddressTest < ActiveSupport::TestCase
     assert_equal "", address.to_s
   end
 
+  test "accepts alphanumeric postal codes" do
+    address = addresses(:one)
+    address.update!(postal_code: "SW1A 2AA")
+    assert_equal "SW1A 2AA", address.reload.postal_code
+  end
+
   test "can strip extras commas and spaces" do
     address = Address.new(
       line1: "123 Main St ,",