feat: add in-memory database support via appsettings UseInMemoryDb flag

- Introduced conditional EF Core configuration to support InMemoryDatabase for testing or lightweight runs.
- Added `UseInMemoryDb` config flag read from appsettings.
- Retained SQL Server as the default when the flag is false.
- Added missing Quartz namespace import.

EnvelopeGenerator.Finalizer/appsettings.Database.json

@@ -17,5 +17,6 @@
     "EnvelopeReceiverReadOnly": [ "TBSIG_ENVELOPE_RECEIVER_READ_ONLY_UPD" ],
     "Receiver": [],
     "EmailTemplate": [ "TBSIG_EMAIL_TEMPLATE_AFT_UPD" ]
-  }
+  },
+  "UseInMemoryDb": true
 }