Replace SendGrid SMTP with Resend for transactional email

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dadachi

Gemfile

@@ -60,6 +60,7 @@ gem "valid_email2"
 gem "importmap-rails"
 gem "tailwindcss-rails", "~> 3.0"
 gem "rack-attack"
+gem "resend"
 # Fix LoadError: cannot load such file -- csv
 gem "csv", "~> 3.3"
 

Gemfile.lock

@@ -177,6 +177,10 @@ GEM
     globalid (1.3.0)
       activesupport (>= 6.1)
     hashdiff (1.2.1)
+    httparty (0.24.2)
+      csv
+      mini_mime (>= 1.0.0)
+      multi_xml (>= 0.5.2)
     i18n (1.14.8)
       concurrent-ruby (~> 1.0)
     image_processing (1.14.0)
@@ -244,6 +248,8 @@ GEM
       stimulus-rails
       turbo-rails
     msgpack (1.8.0)
+    multi_xml (0.8.1)
+      bigdecimal (>= 3.1, < 5)
     net-imap (0.6.3)
       date
       net-protocol
@@ -353,6 +359,8 @@ GEM
     regexp_parser (2.11.3)
     reline (0.6.3)
       io-console (~> 0.5)
+    resend (1.0.1)
+      httparty (>= 0.21.0)
     responders (3.2.0)
       actionpack (>= 7.0)
       railties (>= 7.0)
@@ -507,6 +515,7 @@ DEPENDENCIES
   rack-attack
   rack-cors
   rails (~> 8.1)
+  resend
   rubocop-rails-omakase
   seed-fu (~> 2.3)
   selenium-webdriver (>= 4.20.1)

config/initializers/mail.rb

@@ -6,20 +6,7 @@
     ActionMailer::Base.default_url_options[:host] = ConfigSettings.app.domain
     ActionMailer::Base.default_url_options[:protocol] = "https"
 
-    shared_settings = {
-      port: 587,
-      authentication: :plain,
-      enable_starttls_auto: true,
-      domain: ConfigSettings.site.domain
-    }
-
-    settings = {
-      address: Rails.application.credentials.dig(:smtp, :host),
-      domain: Rails.application.credentials.dig(:smtp, :domain),
-      user_name: Rails.application.credentials.dig(:smtp, :username),
-      password: Rails.application.credentials.dig(:smtp, :password)
-    }.merge(shared_settings)
-
-    ActionMailer::Base.smtp_settings.merge!(settings)
+    ActionMailer::Base.delivery_method = :resend
+    Resend.api_key = Rails.application.credentials.dig(:resend, :api_key)
   end
 end

lib/templates/rails/credentials/credentials.yml.tt

@@ -7,9 +7,6 @@ active_record_encryption:
   deterministic_key: <%= SecureRandom.alphanumeric(32) %>
   key_derivation_salt: <%= SecureRandom.alphanumeric(32) %>
 
-# SMTP
-smtp:
-  host:
-  domain:
-  username:
-  password:
+# Resend
+resend:
+  api_key: