Refactor authentication schemes for sender and receiver

Updated `AuthScheme` to introduce a distinct `Sender` scheme
and renamed the `Receiver` scheme for clarity. Updated
`Program.cs` to use the new `Sender` scheme in JWT
authentication and explicitly associate authentication
schemes with `Sender` and `Receiver` policies. Removed the
deprecated `AuthPolicy.ReceiverTFA` policy. These changes
improve the separation and maintainability of authentication
and authorization logic.

EnvelopeGenerator.API/AuthScheme.cs

@@ -8,5 +8,10 @@ public static class AuthScheme
     /// <summary>
     /// Scheme name used for per-envelope receiver JWT authentication.
     /// </summary>
-    public const string Receiver = "EnvelopeGenerator.API.EnvelopeReceiverJwt";
+    public const string Receiver = "EnvelopeGenerator.API.ReceiverJWT";
+
+    /// <summary>
+    /// Scheme name used for per-envelope sender JWT authentication.
+    /// </summary>
+    public const string Sender = "EnvelopeGenerator.API.SenderJWT";
 }