Consolidate district migrations into single migration with unique index

Author: Mohamed-RPF

db/migrate/20251120145032_add_district_fields_to_schools.rb

@@ -3,6 +3,6 @@ def change
     add_column :schools, :district_name, :string
     add_column :schools, :district_nces_id, :string
 
-    add_index :schools, :district_nces_id
+    add_index :schools, :district_nces_id, unique: true
   end
 end