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/services/ticket_pdf_generator.rb

@@ -2,7 +2,13 @@ require "prawn"
 require "prawn/qrcode"
 require "rqrcode"
 
+# PDF ticket generator service using Prawn
+#
+# Generates PDF tickets with QR codes for event entry validation
+# Includes event details, venue information, and unique QR code for each ticket
 class TicketPdfGenerator
+  # Suppress Prawn's internationalization warning for built-in fonts
+  Prawn::Fonts::AFM.hide_m17n_warning = true
   attr_reader :ticket
 
   def initialize(ticket)