fix: Backup-Download (0 Bytes + fehlender Dateiname)

- api.js: credentials: 'include' hinzugefügt (Auth-Cookie)
- BackupEndpoint.cs: Task<IResult> → Task mit direktem Response-Handling
  (ASP.NET hat IResult nicht korrekt verarbeitet → leerer Body)

Endpoints/BackupEndpoint.cs

@@ -2,6 +2,7 @@ using System;
 using System.Diagnostics;
 using System.IO;
 using System.Linq;
+using System.Threading.Tasks;
 using Gott.Extensions;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
@@ -16,14 +17,23 @@ public static class BackupEndpoint
         app.MapPost("/api/backup", HandleBackup);
     }
 
-    private static async System.Threading.Tasks.Task<IResult> HandleBackup(HttpContext ctx)
+    private static async Task HandleBackup(HttpContext ctx)
     {
         if (!ctx.IsAdmin())
-            return Results.Forbid();
+        {
+            ctx.Response.StatusCode = 403;
+            return;
+        }
+
         var dataDir = Path.Combine(AppContext.BaseDirectory, "data");
 
         if (!Directory.Exists(dataDir))
-            return Results.NotFound(new { error = "Data-Ordner nicht gefunden" });
+        {
+            ctx.Response.StatusCode = 404;
+            ctx.Response.ContentType = "application/json";
+            await ctx.Response.WriteAsync("{\"error\":\"Data-Ordner nicht gefunden\"}");
+            return;
+        }
 
         var backupDir = Path.Combine(dataDir, "backup");
         Directory.CreateDirectory(backupDir);
@@ -53,16 +63,27 @@ public static class BackupEndpoint
         using (var process = Process.Start(psi))
         {
             if (process == null)
-                return Results.StatusCode(500);
+            {
+                ctx.Response.StatusCode = 500;
+                return;
+            }
 
             var stderr = await process.StandardError.ReadToEndAsync();
             await process.WaitForExitAsync();
 
             if (process.ExitCode != 0)
-                return Results.Problem($"tar fehlgeschlagen: {stderr}");
+            {
+                ctx.Response.StatusCode = 500;
+                ctx.Response.ContentType = "application/json";
+                await ctx.Response.WriteAsync($"{{\"error\":\"tar fehlgeschlagen: {stderr}\"}}");
+                return;
+            }
         }
 
         var bytes = await File.ReadAllBytesAsync(archivePath);
-        return Results.File(bytes, "application/gzip", archiveName);
+        ctx.Response.StatusCode = 200;
+        ctx.Response.ContentType = "application/gzip";
+        ctx.Response.Headers["Content-Disposition"] = $"attachment; filename=\"{archiveName}\"";
+        await ctx.Response.Body.WriteAsync(bytes);
     }
 }