Fix Swashbuckle 6.9 OpenAPI generation for IFormFile upload endpoint

Swashbuckle 6.9+ requires IFormFile to be wrapped in a class when used
with [FromForm] rather than declared as a bare action parameter. Introduce
DatabaseImportRequest to satisfy this requirement and unblock the OpenAPI
guardrail CI check.

backend/src/Taskdeck.Api/Contracts/DatabaseImportRequest.cs

@@ -0,0 +1,10 @@
+namespace Taskdeck.Api.Contracts;
+
+/// <summary>
+/// Wraps the IFormFile upload for database import. Using a class wrapper is required
+/// by Swashbuckle 6.9+ to correctly generate the OpenAPI schema for file uploads.
+/// </summary>
+public sealed class DatabaseImportRequest
+{
+    public IFormFile? File { get; set; }
+}

backend/src/Taskdeck.Api/Controllers/ExportController.cs

@@ -1,6 +1,7 @@
 using System.Text.Json;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
+using Taskdeck.Api.Contracts;
 using Taskdeck.Api.Extensions;
 using Taskdeck.Application.DTOs;
 using Taskdeck.Application.Interfaces;
@@ -84,11 +85,13 @@ public async Task<IActionResult> ExportDatabase()
 
     [HttpPost("import/database")]
     [Consumes("multipart/form-data")]
-    public async Task<IActionResult> ImportDatabase([FromForm] IFormFile? file)
+    public async Task<IActionResult> ImportDatabase([FromForm] DatabaseImportRequest request)
     {
         if (!TryGetCurrentUserId(out var userId, out var errorResult))
             return errorResult!;
 
+        var file = request.File;
+
         if (file is null)
         {
             return Result