feat: questpdf reports for contrato, cotizacion, arqueo

- ReportService with 3 QuestPDF report generators
- Contrato PDF with header, student info, and course table
- Cotización PDF with customer info, total, and detail table
- Arqueo PDF with cashier summary and payment method breakdowns
- ReportController with endpoints for PDF download
- Registered in DI

backend/src/Ventas.API/Program.cs

@@ -34,6 +34,7 @@ builder.Services.AddScoped<AlumnoService>(sp =>
     new AlumnoService(connectionString));
 builder.Services.AddScoped<InformeService>(sp =>
     new InformeService(connectionString));
+builder.Services.AddScoped<ReportService>();
 
 var jwtSecret = builder.Configuration["Jwt:Secret"] ?? "default-dev-secret-change-in-production";
 var jwtExpiration = int.Parse(builder.Configuration["Jwt:ExpirationMinutes"] ?? "30");