user upgrades: upgrade to new Stripe checkout system.

This upgrades from the legacy version of Stripe's checkout system to the
new version:

> The legacy version of Checkout presented customers with a modal dialog
> that collected card information, and returned a token or a source to
> your website. In contrast, the new version of Checkout is a smart
> payment page hosted by Stripe that creates payments or subscriptions. It
> supports Apple Pay, Dynamic 3D Secure, and many other features.

Basic overview of the new system:

* We send the user to a checkout page on Stripe.
* Stripe collects payment and sends us a webhook notification when the
  order is complete.
* We receive the webhook notification and upgrade the user.

Docs:

* https://stripe.com/docs/payments/checkout
* https://stripe.com/docs/payments/checkout/migration#client-products
* https://stripe.com/docs/payments/handling-payment-events
* https://stripe.com/docs/payments/checkout/fulfill-orders

app/logical/user_upgrade.rb

@@ -1,13 +1,145 @@
 class UserUpgrade
+  attr_reader :recipient, :purchaser, :level
+
+  def self.stripe_publishable_key
+    Danbooru.config.stripe_publishable_key
+  end
+
+  def self.stripe_webhook_secret
+    Danbooru.config.stripe_webhook_secret
+  end
+
   def self.gold_price
     2000
   end
 
   def self.platinum_price
-    4000
+    2 * gold_price
   end
 
-  def self.upgrade_price
-    2000
+  def self.gold_to_platinum_price
+    platinum_price - gold_price
+  end
+
+  def initialize(recipient:, purchaser:, level:)
+    @recipient, @purchaser, @level = recipient, purchaser, level.to_i
+  end
+
+  def upgrade_type
+    if level == User::Levels::GOLD && recipient.level == User::Levels::MEMBER
+      :gold_upgrade
+    elsif level == User::Levels::PLATINUM && recipient.level == User::Levels::MEMBER
+      :platinum_upgrade
+    elsif level == User::Levels::PLATINUM && recipient.level == User::Levels::GOLD
+      :gold_to_platinum_upgrade
+    else
+      raise ArgumentError, "Invalid upgrade"
+    end
+  end
+
+  def upgrade_price
+    case upgrade_type
+    when :gold_upgrade
+      UserUpgrade.gold_price
+    when :platinum_upgrade
+      UserUpgrade.platinum_price
+    when :gold_to_platinum_upgrade
+      UserUpgrade.gold_to_platinum_price
+    end
+  end
+
+  def upgrade_description
+    case upgrade_type
+    when :gold_upgrade
+      "Upgrade to Gold"
+    when :platinum_upgrade
+      "Upgrade to Platinum"
+    when :gold_to_platinum_upgrade
+      "Upgrade Gold to Platinum"
+    end
+  end
+
+  def is_gift?
+    recipient != purchaser
+  end
+
+  def process_upgrade!
+    recipient.with_lock do
+      upgrade_recipient!
+    end
+  end
+
+  def upgrade_recipient!
+    recipient.promote_to!(level, User.system, is_upgrade: true)
+  end
+
+  concerning :StripeMethods do
+    def create_checkout
+      Stripe::Checkout::Session.create(
+        mode: "payment",
+        success_url: Routes.user_upgrade_url(user_id: recipient.id),
+        cancel_url: Routes.new_user_upgrade_url(user_id: recipient.id),
+        client_reference_id: "user_#{purchaser.id}",
+        customer_email: recipient.email_address&.address,
+        payment_method_types: ["card"],
+        line_items: [{
+          price_data: {
+            unit_amount: upgrade_price,
+            currency: "usd",
+            product_data: {
+              name: upgrade_description,
+            },
+          },
+          quantity: 1,
+        }],
+        metadata: {
+          purchaser_id: purchaser.id,
+          recipient_id: recipient.id,
+          purchaser_name: purchaser.name,
+          recipient_name: recipient.name,
+          upgrade_type: upgrade_type,
+          is_gift: is_gift?,
+          level: level,
+        },
+      )
+    end
+
+    class_methods do
+      def register_webhook
+        webhook = Stripe::WebhookEndpoint.create({
+          url: Routes.webhook_user_upgrade_url(source: "stripe"),
+          enabled_events: [
+            "payment_intent.created",
+            "payment_intent.payment_failed",
+            "checkout.session.completed",
+          ],
+        })
+
+        webhook.secret
+      end
+
+      def receive_webhook(request)
+        event = build_event(request)
+
+        if event.type == "checkout.session.completed"
+          checkout_session_completed(event)
+        end
+      end
+
+      def build_event(request)
+        payload = request.body.read
+        signature = request.headers["Stripe-Signature"]
+        Stripe::Webhook.construct_event(payload, signature, stripe_webhook_secret)
+      end
+
+      def checkout_session_completed(event)
+        recipient = User.find(event.data.object.metadata.recipient_id)
+        purchaser = User.find(event.data.object.metadata.purchaser_id)
+        level = event.data.object.metadata.level
+
+        user_upgrade = UserUpgrade.new(recipient: recipient, purchaser: purchaser, level: level)
+        user_upgrade.process_upgrade!
+      end
+    end
   end
 end