Add contestant categories to contests

db/migrate/20250714115208_create_categories.rb

@@ -0,0 +1,15 @@
+class CreateCategories < ActiveRecord::Migration[8.0]
+  def change
+    create_table :categories do |t|
+      t.string :name
+      t.belongs_to :contest, null: false, foreign_key: true
+
+      t.timestamps
+    end
+
+    create_join_table :categories, :contestants do |t|
+      t.index :category_id
+      t.index :contestant_id
+    end
+  end
+end