refactor: Improve code quality and add comprehensive documentation

- Remove unused create_stripe_session method from TicketsController
- Replace hardcoded API key with environment variable for security
- Fix typo in ApplicationHelper comment
- Improve User model validation constraints for better UX
- Add comprehensive YARD-style documentation across models, controllers, services, and helpers
- Enhance error handling in cleanup jobs with proper exception handling
- Suppress Prawn font warnings in PDF generator
- Update refactoring summary with complete change documentation

All tests pass (200 tests, 454 assertions, 0 failures)
RuboCop style issues resolved automatically

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/jobs/cleanup_expired_drafts_job.rb

@@ -1,15 +1,33 @@
+# Background job to clean up expired draft tickets
+#
+# This job runs periodically to find and expire draft tickets that have
+# passed their expiry time (typically 30 minutes after creation).
+# Should be scheduled via cron or similar scheduling system.
 class CleanupExpiredDraftsJob < ApplicationJob
   queue_as :default
 
+  # Find and expire all draft tickets that have passed their expiry time
+  #
+  # Uses find_each to process tickets in batches to avoid memory issues
+  # with large datasets. Continues processing even if individual tickets fail.
   def perform
     expired_count = 0
 
+    # Process expired draft tickets in batches
     Ticket.expired_drafts.find_each do |ticket|
-      Rails.logger.info "Expiring draft ticket #{ticket.id} for user #{ticket.user.id}"
-      ticket.expire_if_overdue!
-      expired_count += 1
+      begin
+        Rails.logger.info "Expiring draft ticket #{ticket.id} for user #{ticket.user.id}"
+        ticket.expire_if_overdue!
+        expired_count += 1
+      rescue => e
+        # Log error but continue processing other tickets
+        Rails.logger.error "Failed to expire ticket #{ticket.id}: #{e.message}"
+        next
+      end
     end
 
+    # Log summary if any tickets were processed
     Rails.logger.info "Expired #{expired_count} draft tickets" if expired_count > 0
+    Rails.logger.info "No expired draft tickets found" if expired_count == 0
   end
 end