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/environments/production.rb

@@ -49,9 +49,8 @@ Rails.application.configure do
   # Replace the default in-process memory cache store with a durable alternative.
   config.cache_store = :solid_cache_store
 
-  # Replace the default in-process and non-durable queuing backend for Active Job.
-  config.active_job.queue_adapter = :solid_queue
-  config.solid_queue.connects_to = { database: { writing: :queue } }
+  # Use async processor for background jobs (modify as needed for production)
+  config.active_job.queue_adapter = :async
 
   # Ignore bad email addresses and do not raise email delivery errors.
   # Set this to true and configure the email server for immediate delivery to raise delivery errors.