create basic backend with login

main-server/app/models/application_record.rb

@@ -0,0 +1,3 @@
+class ApplicationRecord < ActiveRecord::Base
+  primary_abstract_class
+end

main-server/app/models/user.rb

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+class User < ApplicationRecord
+  has_secure_password validations: false
+
+  has_many :sessions, dependent: :destroy
+  has_one_attached :avatar
+
+  normalizes :nickname, with: ->(n) { n.strip }
+
+  validates :nickname, presence: true, uniqueness: { case_sensitive: false }, length: { minimum: 3, maximum: 24 }
+  validates :password, presence: true, length: { minimum: 6 }, if: -> { new_record? || password_digest_changed? }
+  validate :avatar_content_type, if: -> { avatar.attached? }
+
+  private
+    def avatar_content_type
+      unless avatar.blob.content_type.in?(%w[image/png image/jpeg image/gif image/webp])
+        errors.add(:avatar, "must be a PNG, JPEG, GIF or WebP image")
+      end
+    end
+end