Refactor email delivery and background jobs system

- Switch from SolidQueue to async job processor for simpler background job handling
- Remove SolidQueue gem and related configuration files
- Add letter_opener gem for development email preview
- Fix invitation email template issues (invitation_login_token method and route helper)
- Configure SMTP settings via environment variables in application.rb
- Add email delivery configuration banner on admin users page
- Improve admin users page with inline action buttons and SMTP configuration warnings
- Update development and production environments to use async processor
- Add helper methods to detect SMTP configuration and filter out localhost settings

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/helpers/application_helper.rb

@@ -1,2 +1,22 @@
 module ApplicationHelper
+  def smtp_configured?
+    return true if Rails.env.test?
+
+    smtp_address = ENV["SMTP_ADDRESS"]
+    smtp_port = ENV["SMTP_PORT"]
+
+    smtp_address.present? &&
+    smtp_port.present? &&
+    smtp_address != "localhost" &&
+    !smtp_address.start_with?("127.0.0.1") &&
+    !smtp_address.start_with?("localhost")
+  end
+
+  def email_delivery_method
+    if Rails.env.development?
+      ActionMailer::Base.delivery_method
+    else
+      :smtp
+    end
+  end
 end