Add envelope receiver token endpoint to AuthController

Refactor `AuthController` to include `IMediator` dependency and
introduce a new `CreateTokenForEnvelopeReceiver` API endpoint
to handle envelope receiver authentication.

- Updated `using` directives to remove unused namespaces and
  add required ones for new functionality.
- Added `ReceiverLogin` model to represent envelope receiver
  login credentials.
- Implemented `ReadEnvelopeReceiverSecretQuery` to validate
  access codes for envelope receivers.
- Cleaned up unused fields and dependencies in `AuthController`.

src/DigitalData.Auth.API/Models/ReceiverLogin.cs

@@ -0,0 +1,4 @@
+namespace DigitalData.Auth.API.Models
+{
+    public record ReceiverLogin(string AccessCode);
+}