feat: standalone ExcelToPdfService - ASP.NET Core 8 + Syncfusion

- Services/ExcelToPdfService.cs: core conversion logic (XlsIORenderer)
- Controllers/ExcelToPdfController.cs: REST API endpoints
  * POST /api/ExcelToPdf/convert (file upload)
  * POST /api/ExcelToPdf/convert-by-path (server path)
  * GET  /api/ExcelToPdf/health
- Program.cs: minimal API setup, 50MB upload limit
- appsettings.json: port 5200, Syncfusion license
- Syncfusion 21.1.37 (XlsIO + Pdf + XlsIORenderer)
- Binary signed: Azure Trusted Signing (Private Trust)

Program.cs

@@ -0,0 +1,30 @@
+using ExcelToPdfService.Services;
+using Syncfusion.Licensing;
+
+var builder = WebApplication.CreateBuilder(args);
+
+// Syncfusion license
+SyncfusionLicenseProvider.RegisterLicense(
+    builder.Configuration["Syncfusion:LicenseKey"]
+    ?? "MjExMDkzMEAzMjMxMmUzMTJlMzMzNWd1RGM1NVFmUUMzWmZlN0dCc3NadUJyM1RUYVh1SHVSS1B2Tzdwa0NhcE09");
+
+// Services
+builder.Services.AddControllers();
+builder.Services.AddScoped<IExcelToPdfService, ExcelToPdfService.Services.ExcelToPdfService>();
+
+// Allow large file uploads (50 MB)
+builder.Services.Configure<Microsoft.AspNetCore.Http.Features.FormOptions>(o =>
+{
+    o.MultipartBodyLengthLimit = 50 * 1024 * 1024;
+});
+builder.WebHost.ConfigureKestrel(o =>
+{
+    o.Limits.MaxRequestBodySize = 50 * 1024 * 1024;
+});
+
+var app = builder.Build();
+
+app.UseHttpsRedirection();
+app.MapControllers();
+
+app.Run();