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/puma.rb

@@ -34,8 +34,7 @@ port ENV.fetch("PORT", 3000)
 # Allow puma to be restarted by `bin/rails restart` command.
 plugin :tmp_restart
 
-# Run the Solid Queue supervisor inside of Puma for single-server deployments.
-plugin :solid_queue if ENV["SOLID_QUEUE_IN_PUMA"]
+# Solid Queue plugin removed - now using async processor
 
 # Specify the PID file. Defaults to tmp/pids/server.pid in development.
 # In other environments, only set the PID file if requested.