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/Controllers/DocumentsController.cs

@@ -0,0 +1,38 @@
+using DigitalData.TaskFlow.Application.Features.Documents.Queries.GetDocument;
+using DigitalData.TaskFlow.Domain.Common;
+using DigitalData.TaskFlow.Domain.Documents;
+using MediatR;
+using Microsoft.AspNetCore.Mvc;
+
+namespace DigitalData.TaskFlow.API.Controllers;
+
+/// <summary>
+/// Provides document binary download endpoints.
+/// </summary>
+[ApiController]
+[Route("api/documents")]
+public sealed class DocumentsController(ISender sender) : ControllerBase
+{
+    /// <summary>
+    /// Returns binary document content by document identifier.
+    /// </summary>
+    /// <param name="id">Document identifier.</param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <returns>Binary stream when available.</returns>
+    [HttpGet("{id}")]
+    [ProducesResponseType(typeof(FileContentResult), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ApiEnvelope<object>), StatusCodes.Status400BadRequest)]
+    [ProducesResponseType(typeof(ApiEnvelope<object>), StatusCodes.Status500InternalServerError)]
+    public async Task<IActionResult> GetAsync([FromRoute] string id, CancellationToken cancellationToken)
+    {
+        var response = await sender.Send(new GetDocumentQuery(id), cancellationToken);
+        var data = response.Data;
+
+        if (data is null)
+        {
+            return NotFound(response);
+        }
+
+        return File(data.Content, data.ContentType, data.FileName);
+    }
+}