Refactor JWT auth scheme configuration

Replaced hardcoded per-envelope receiver JWT auth scheme string with a new `AuthScheme` static class containing a `Receiver` constant. Updated `Program.cs` to use `AuthScheme.Receiver` for authentication and policy configuration. Removed redundant comments and unused constants. Added necessary `using` directive for `AuthScheme`.

EnvelopeGenerator.API/AuthScheme.cs

@@ -0,0 +1,12 @@
+namespace EnvelopeGenerator.API;
+
+/// <summary>
+/// 
+/// </summary>
+public static class AuthScheme
+{
+    /// <summary>
+    /// Scheme name used for per-envelope receiver JWT authentication.
+    /// </summary>
+    public const string Receiver = "EnvelopeGenerator.API.EnvelopeReceiverJwt";
+}