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>

config/application.rb

@@ -23,5 +23,18 @@ module Clinch
     #
     # config.time_zone = "Central Time (US & Canada)"
     # config.eager_load_paths << Rails.root.join("extras")
+
+      # Configure SMTP settings using environment variables
+    config.action_mailer.delivery_method = :smtp
+    config.action_mailer.smtp_settings = {
+      address: ENV.fetch('SMTP_ADDRESS', 'localhost'),
+      port: ENV.fetch('SMTP_PORT', 587),
+      domain: ENV.fetch('SMTP_DOMAIN', 'localhost'),
+      user_name: ENV.fetch('SMTP_USER_NAME', nil),
+      password: ENV.fetch('SMTP_PASSWORD', nil),
+      authentication: ENV.fetch('SMTP_AUTHENTICATION', 'plain').to_sym,
+      enable_starttls_auto: ENV.fetch('SMTP_STARTTLS_AUTO', 'true') == 'true',
+      openssl_verify_mode: OpenSSL::SSL::VERIFY_PEER
+    }
   end
 end