Add migration to backfill the district_name field for US schools wher… (#661)

## Status

-
[Closes](RaspberryPiFoundation/digital-editor-issues#1092)

## What's changed?

- Add migration to backfill the `district_name` field on schools where
the `country_code` is US and `district_name` is null or empty

Author: jamiebenstead

db/migrate/20260121103308_backfill_district_name_for_us_schools.rb

@@ -0,0 +1,15 @@
+class BackfillDistrictNameForUsSchools < ActiveRecord::Migration[7.2]
+  def up
+    School
+      .where(country_code: "US")
+      .where("district_name IS NULL OR BTRIM(district_name) = ''")
+      .update_all(district_name: "District Unknown")
+  end
+
+  def down
+    School
+      .where(country_code: "US")
+      .where(district_name: "District Unknown")
+      .update_all(district_name: nil)
+  end
+end