validate order's email against existing users

see #3

app/models/boutique/order.rb

@@ -43,6 +43,11 @@ class Boutique::Order < Boutique::ApplicationRecord
             unless: :pending?
 
   validate :validate_primary_address
+  validate :validate_email_not_already_registered, unless: :pending?
+
+  validates :primary_address,
+            presence: true,
+            if: :requires_address_and_not_pending?
 
   aasm timestamps: true do
     state :pending, initial: true
@@ -180,8 +185,16 @@ def validate_primary_address
         errors.add(:primary_address, :blank)
       end
     end
-end
 
+    def validate_email_not_already_registered
+      return if email.nil?
+      return if user.present?
+
+      if Folio::User.where(email:).exists?
+        errors.add(:email, :already_registered)
+      end
+    end
+end
 # == Schema Information
 #
 # Table name: boutique_orders

config/locales/activerecord.cs.yml

@@ -50,3 +50,8 @@ cs:
             base:
               missing_master_variant: Nemá vybranou hlavní variantu.
               too_many_master_variants: Má vybrano více hlavních variant.
+
+        boutique/order:
+          attributes:
+            email:
+              already_registered: je již registrovaný