fix(BurnPdfCommand): improve JSON serialization and null safety in Signature entity and BurnPdfCommandHandler

- Added System.Text.Json.Serialization namespace and [JsonIgnore] attributes to Top and Left properties in Signature entity for .NET builds
- Applied null-forgiving operator (!) and null-coalescing defaults for frame.X and frame.Y in BurnPdfCommandHandler
- Improved runtime safety and prevented unintended JSON serialization of computed properties

EnvelopeGenerator.Finalizer/appsettings.Database.json

@@ -18,5 +18,5 @@
     "Receiver": [],
     "EmailTemplate": [ "TBSIG_EMAIL_TEMPLATE_AFT_UPD" ]
   },
-  "UseInMemoryDb": true
+  "UseInMemoryDb": false
 }