Add envelope-receiver login endpoint support

Added `AddEnvelopeReceiverLoginOperation` to `AuthProxyDocumentFilter` to define a new Swagger operation for the `/api/Auth/envelope-receiver/{key}` endpoint. Introduced the `EnvelopeReceiverLogin` record for the request body. Updated `yarp.json` to configure routing for the new endpoint, ensuring the `cookie` query parameter is always set to `true`. Modified `Apply` to include the new operation in Swagger documentation.

EnvelopeGenerator.API/Documentation/AuthProxyDocumentFilter.cs

@@ -16,6 +16,12 @@ public sealed class AuthProxyDocumentFilter : IDocumentFilter
     /// <param name="swaggerDoc"></param>
     /// <param name="context"></param>
     public void Apply(OpenApiDocument swaggerDoc, DocumentFilterContext context)
+    {
+        AddLoginOperation(swaggerDoc, context);
+        AddEnvelopeReceiverLoginOperation(swaggerDoc, context);
+    }
+
+    private static void AddLoginOperation(OpenApiDocument swaggerDoc, DocumentFilterContext context)
     {
         const string path = "/api/auth";
 
@@ -67,4 +73,51 @@ public sealed class AuthProxyDocumentFilter : IDocumentFilter
             }
         };
     }
+
+    private static void AddEnvelopeReceiverLoginOperation(OpenApiDocument swaggerDoc, DocumentFilterContext context)
+    {
+        const string path = "/api/Auth/envelope-receiver/{key}";
+
+        var bodySchema = context.SchemaGenerator.GenerateSchema(typeof(EnvelopeReceiverLogin), context.SchemaRepository);
+
+        var operation = new OpenApiOperation
+        {
+            Summary = "Envelope receiver login (auth-hub proxy)",
+            Description = "Proxies the envelope receiver login to the auth service. " +
+                          "The `cookie` query parameter is always forwarded as `true` so the auth service sets the per-envelope cookie automatically.",
+            Tags = [new() { Name = "Auth" }],
+            Parameters =
+            {
+                new OpenApiParameter
+                {
+                    Name = "key",
+                    In = ParameterLocation.Path,
+                    Required = true,
+                    Schema = new OpenApiSchema { Type = "string" },
+                    Description = "The unique envelope receiver key."
+                }
+            },
+            RequestBody = new OpenApiRequestBody
+            {
+                Required = false,
+                Content =
+                {
+                    ["multipart/form-data"] = new OpenApiMediaType { Schema = bodySchema }
+                }
+            },
+            Responses =
+            {
+                ["200"] = new OpenApiResponse { Description = "OK – per-envelope cookie set by auth service." },
+                ["401"] = new OpenApiResponse { Description = "Unauthorized – invalid or missing access code." }
+            }
+        };
+
+        swaggerDoc.Paths[path] = new OpenApiPathItem
+        {
+            Operations =
+            {
+                [OperationType.Post] = operation
+            }
+        };
+    }
 }