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>

Gemfile

@@ -34,9 +34,8 @@ gem "jwt", "~> 3.1"
 # Windows does not include zoneinfo files, so bundle the tzinfo-data gem
 gem "tzinfo-data", platforms: %i[ windows jruby ]
 
-# Use the database-backed adapters for Rails.cache, Active Job, and Action Cable
+# Use the database-backed adapters for Rails.cache and Action Cable
 gem "solid_cache"
-gem "solid_queue"
 gem "solid_cable"
 
 # Reduces boot times through caching; required in config/boot.rb
@@ -68,6 +67,9 @@ end
 group :development do
   # Use console on exceptions pages [https://github.com/rails/web-console]
   gem "web-console"
+
+  # Preview emails in browser instead of sending them
+  gem "letter_opener"
 end
 
 group :test do