Add completions

db/migrate/20250320082658_create_completions.rb

@@ -0,0 +1,11 @@
+class CreateCompletions < ActiveRecord::Migration[8.0]
+  def change
+    create_table :completions do |t|
+      t.integer :time_seconds
+      t.belongs_to :contestant, null: false, foreign_key: true
+      t.belongs_to :puzzle, null: false, foreign_key: true
+
+      t.timestamps
+    end
+  end
+end