Add workflow and document APIs, middleware, and Swagger

Introduced `CloseWorkflowRequest` for closing workflows.
Added `DocumentsController` for document downloads and
`WorkflowsController` for workflow retrieval and closing.

Implemented `ExceptionHandlingMiddleware` for global exception
handling with structured API responses.

Enhanced Swagger configuration with metadata and production
support. Added `Scalar.AspNetCore` for API reference mapping.

Updated project dependencies and improved API documentation.

src/presentation/DigitalData.TaskFlow.API/Middleware/ExceptionHandlingMiddleware.cs

@@ -0,0 +1,60 @@
+using DigitalData.TaskFlow.Application.Common.Factories;
+using DigitalData.TaskFlow.Domain.Common;
+using DigitalData.TaskFlow.Domain.Enums;
+using FluentValidation;
+using System.Text.Json;
+
+namespace DigitalData.TaskFlow.API.Middleware;
+
+/// <summary>
+/// Handles exceptions globally and returns ticket-compatible API envelopes.
+/// </summary>
+public sealed class ExceptionHandlingMiddleware(RequestDelegate next, ILogger<ExceptionHandlingMiddleware> logger)
+{
+    private static readonly JsonSerializerOptions JsonOptions = new(JsonSerializerDefaults.Web);
+
+    /// <summary>
+    /// Executes the middleware.
+    /// </summary>
+    /// <param name="context">HTTP context.</param>
+    public async Task InvokeAsync(HttpContext context)
+    {
+        try
+        {
+            await next(context);
+        }
+        catch (ValidationException validationException)
+        {
+            logger.LogWarning(validationException, "Validation failed.");
+
+            context.Response.StatusCode = StatusCodes.Status400BadRequest;
+            context.Response.ContentType = "application/json";
+
+            var message = validationException.Errors.FirstOrDefault()?.ErrorMessage
+                          ?? "Ungultiger Request";
+
+            var response = ApiEnvelopeFactory.Create<object?>(null, MetaCode.InvalidRequest, message);
+            await context.Response.WriteAsync(JsonSerializer.Serialize(response, JsonOptions));
+        }
+        catch (NotSupportedException notSupportedException)
+        {
+            logger.LogWarning(notSupportedException, "Operation not supported yet.");
+
+            context.Response.StatusCode = StatusCodes.Status500InternalServerError;
+            context.Response.ContentType = "application/json";
+
+            var response = ApiEnvelopeFactory.Create<object?>(null, MetaCode.InternalError, notSupportedException.Message);
+            await context.Response.WriteAsync(JsonSerializer.Serialize(response, JsonOptions));
+        }
+        catch (Exception exception)
+        {
+            logger.LogError(exception, "Unhandled exception.");
+
+            context.Response.StatusCode = StatusCodes.Status500InternalServerError;
+            context.Response.ContentType = "application/json";
+
+            var response = ApiEnvelopeFactory.Create<object?>(null, MetaCode.InternalError);
+            await context.Response.WriteAsync(JsonSerializer.Serialize(response, JsonOptions));
+        }
+    }
+}