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/recurring.yml

@@ -1,15 +0,0 @@
-# examples:
-#   periodic_cleanup:
-#     class: CleanSoftDeletedRecordsJob
-#     queue: background
-#     args: [ 1000, { batch_size: 500 } ]
-#     schedule: every hour
-#   periodic_cleanup_with_command:
-#     command: "SoftDeletedRecord.due.delete_all"
-#     priority: 2
-#     schedule: at 5am every day
-
-production:
-  clear_solid_queue_finished_jobs:
-    command: "SolidQueue::Job.clear_finished_in_batches(sleep_between_batches: 0.3)"
-    schedule: every hour at minute 12