Add offline model and "new" form/controller

db/migrate/20251029155116_create_offlines.rb

@@ -0,0 +1,10 @@
+class CreateOfflines < ActiveRecord::Migration[8.0]
+  def change
+    create_table :offlines do |t|
+      t.timestamps
+      t.belongs_to :contest, null: false, foreign_key: true
+      t.datetime :start_time, null: false
+      t.datetime :end_time
+    end
+  end
+end