fix: Replace Prawn with Grover for PDF ticket generation

- Replace Prawn PDF generation with Grover (Chrome headless) for better compatibility
- Add HTML-based ticket template with embedded CSS styling
- Implement robust Grover loading with fallback to HTML download
- Add QR code generation methods to Ticket model
- Remove legacy TicketPdfGenerator service and tests
- Update PDF generation in TicketsController with proper error handling

The new implementation provides:
- Better HTML/CSS rendering for ticket layouts
- More reliable PDF generation using Chrome engine
- Fallback mechanism for better user experience
- Cleaner separation of template rendering and PDF conversion

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

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

app/views/tickets/show.pdf.erb

@@ -0,0 +1,14 @@
+<% content_for :title, "Ticket ##{ticket.id}" %>
+
+<div style="font-family: Arial, sans-serif; max-width: 350px; margin: 20px auto; padding: 20px; border: 1px solid #ccc;">
+  <div style="text-align: center;">
+    <h1 style="color: #2D1B69;">ApéroNight</h1>
+  </div>
+  <h2><%= ticket.event.name %></h2>
+  <p>Ticket Holder: <%= ticket.first_name %> <%= ticket.last_name %></p>
+  <p>Ticket Type: <%= ticket.ticket_type.name %></p>
+  <p>Price: €<%= ticket.price_euros %></p>
+  <div style="text-align: center; margin-top: 20px;">
+    <%= raw ticket.generate_qr_svg %>
+  </div>
+</div>