Add Swagger doc filter for /api/auth proxy login endpoint

Introduced AuthProxyDocumentFilter to programmatically document the POST /api/auth proxy login endpoint in Swagger. The filter defines request body schemas, example values, query parameter, and response codes. Registered the filter in Program.cs for OpenAPI generation.

EnvelopeGenerator.API/Program.cs

@@ -106,6 +106,8 @@ try
         {
             options.IncludeXmlComments(xmlFile);
         }
+
+        options.DocumentFilter<EnvelopeGenerator.API.Documentation.AuthProxyDocumentFilter>();
     });
     builder.Services.AddOpenApi();