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/controllers/tickets_controller.rb

@@ -73,34 +73,4 @@ class TicketsController < ApplicationController
     Rails.logger.error "TicketsController#set_event - Event not found with ID: #{event_id}"
     redirect_to events_path, alert: "Événement non trouvé"
   end
-
-
-  def create_stripe_session
-    line_items = @tickets.map do |ticket|
-      {
-        price_data: {
-          currency: "eur",
-          product_data: {
-            name: "#{@event.name} - #{ticket.ticket_type.name}",
-            description: ticket.ticket_type.description
-          },
-          unit_amount: ticket.price_cents
-        },
-        quantity: 1
-      }
-    end
-
-    Stripe::Checkout::Session.create(
-      payment_method_types: [ "card" ],
-      line_items: line_items,
-      mode: "payment",
-      success_url: payment_success_url + "?session_id={CHECKOUT_SESSION_ID}",
-      cancel_url: payment_cancel_url,
-      metadata: {
-        event_id: @event.id,
-        user_id: current_user.id,
-        ticket_ids: @tickets.pluck(:id).join(",")
-      }
-    )
-  end
 end