upgrades: rename stripe_id to transaction_id

* Rename the stripe_id column to transaction_id.
* Add a new payment_processor column to identity the processor used for
  this transaction (and hence, which backend system the transaction_id is for).

app/logical/payment_transaction.rb

@@ -5,7 +5,7 @@
 # @see app/logical/payment_transaction/stripe.rb
 class PaymentTransaction
   attr_reader :user_upgrade
-  delegate :recipient, :purchaser, :upgrade_type, :pending?, :stripe_id, to: :user_upgrade
+  delegate :recipient, :purchaser, :upgrade_type, :pending?, :transaction_id, to: :user_upgrade
 
   def initialize(user_upgrade)
     @user_upgrade = user_upgrade

app/logical/payment_transaction/stripe.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class PaymentTransaction::Stripe < PaymentTransaction
-  delegate :stripe_id, to: :user_upgrade
+  delegate :transaction_id, to: :user_upgrade
 
   def self.enabled?
     Danbooru.config.stripe_secret_key.present? && Danbooru.config.stripe_publishable_key.present? && Danbooru.config.stripe_webhook_secret.present?
@@ -36,7 +36,7 @@ class PaymentTransaction::Stripe < PaymentTransaction
       }
     )
 
-    user_upgrade.update!(stripe_id: checkout_session.id)
+    user_upgrade.update!(payment_processor: :stripe, transaction_id: checkout_session.id)
     checkout_session
   end
 
@@ -100,12 +100,12 @@ class PaymentTransaction::Stripe < PaymentTransaction
   end
 
   def receipt_url
-    return nil if pending? || stripe_id.nil?
+    return nil if pending? || transaction_id.nil?
     charge.receipt_url
   end
 
   def payment_url
-    return nil if pending? || stripe_id.nil?
+    return nil if pending? || transaction_id.nil?
     "https://dashboard.stripe.com/payments/#{payment_intent.id}"
   end
 
@@ -118,8 +118,8 @@ class PaymentTransaction::Stripe < PaymentTransaction
   end
 
   private def checkout_session
-    return nil if stripe_id.nil?
-    @checkout_session ||= ::Stripe::Checkout::Session.retrieve(stripe_id)
+    return nil if transaction_id.nil?
+    @checkout_session ||= ::Stripe::Checkout::Session.retrieve(transaction_id)
   end
 
   private def payment_intent

app/models/user_upgrade.rb

@@ -19,6 +19,10 @@ class UserUpgrade < ApplicationRecord
     refunded: 30,
   }
 
+  enum payment_processor: {
+    stripe: 0,
+  }
+
   scope :gifted, -> { where("recipient_id != purchaser_id") }
   scope :self_upgrade, -> { where("recipient_id = purchaser_id") }
 
@@ -85,7 +89,7 @@ class UserUpgrade < ApplicationRecord
   end
 
   def self.search(params)
-    q = search_attributes(params, :id, :created_at, :updated_at, :upgrade_type, :status, :stripe_id, :recipient, :purchaser)
+    q = search_attributes(params, :id, :created_at, :updated_at, :upgrade_type, :status, :transaction_id, :payment_processor, :recipient, :purchaser)
 
     if params[:is_gifted].to_s.truthy?
       q = q.gifted