Add YARP reverse proxy for auth request forwarding

Added the `Yarp.ReverseProxy` package and configured the app to use
YARP for forwarding specific authentication-related API requests
to an external service (`auth-hub`). Updated `Program.cs` to load
YARP configuration from a new `yarp.json` file and added middleware
to map unmatched requests to the reverse proxy.

Replaced old routes and clusters with new routes (`auth-login`,
`auth-envelope-receiver-login`) and a new cluster (`auth-hub`)
pointing to `https://localhost:9090`. Configured route
transformations for path and query parameter adjustments.

These changes improve modularity and scalability by enabling
dynamic reverse proxy configuration and external service
integration.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Program.cs

@@ -32,6 +32,9 @@ try
 {
     var builder = WebApplication.CreateBuilder(args);
 
+    // Load YARP configuration from yarp.json
+    builder.Configuration.AddJsonFile("yarp.json", optional: true, reloadOnChange: true);
+
     builder.Logging.SetMinimumLevel(Microsoft.Extensions.Logging.LogLevel.Trace);
 
     if (!builder.Environment.IsDevelopment())
@@ -53,6 +56,10 @@ try
     builder.Services.AddControllers();
     builder.Services.AddHttpClient();
 
+    // YARP Reverse Proxy (for forwarding auth requests to AuthHub)
+    builder.Services.AddReverseProxy()
+        .LoadFromConfig(builder.Configuration.GetSection("ReverseProxy"));
+
     // HttpContextAccessor needed for SSR HttpClient configuration
     builder.Services.AddHttpContextAccessor();
 
@@ -370,6 +377,9 @@ try
     // API Controllers (map before Blazor routing)
     app.MapControllers();
 
+    // YARP Reverse Proxy - forwards unmatched requests to configured backends
+    app.MapReverseProxy();
+
     // Blazor routing
     app.MapRazorComponents<App>()
         .AddInteractiveServerRenderMode()