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)

appsettings.Development.json

@@ -0,0 +1,8 @@
+{
+  "Logging": {
+    "LogLevel": {
+      "Default": "Information",
+      "Microsoft.AspNetCore": "Warning"
+    }
+  }
+}