create basic backend with login

main-server/db/migrate/20260920183452_create_users.rb

@@ -0,0 +1,11 @@
+class CreateUsers < ActiveRecord::Migration[8.1]
+  def change
+    create_table :users do |t|
+      t.string :nickname, null: false
+      t.string :password_digest, null: false
+
+      t.timestamps
+    end
+    add_index :users, :nickname, unique: true
+  end
+end