feat(worker): parallel multipart uploads for large artifacts

From the studio a single connection to Hetzner nbg1 (188 ms away) is
capped near 0.1 MB/s, although a browser speed test shows 50 Mbps.
Measured: 1 connection 0.13 MB/s, 8 -> 0.64, 24 -> 1.69 MB/s. At the old
speed a ProRes MOV could not finish inside the 30 min request timeout.

- Worker 1.7.2 uploads files >= 16 MB as S3 multipart: 8 MB parts on
  uploadConcurrency connections (config.json, default 16), each part
  retried up to 4 times; aborts the upload if a part keeps failing.
- artifact-presign returns uploadId + per-part presigned URLs when asked
  for multipart; new E20b artifact-complete stitches or aborts.
- Old workers and old servers fall back to the single PUT.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

RenderWorker/VFXReviewWorker/Dtos.cs

@@ -304,6 +304,11 @@ public sealed class PresignResponse
     public string PresignedUrl { get; set; } = "";
     public string Key { get; set; } = "";
     public string Url { get; set; } = "";
+    // Multipart (server ≥ 1.7.2 contract) — absent from older servers, which
+    // only ever return PresignedUrl.
+    public string? UploadId { get; set; }
+    public long PartSize { get; set; }
+    public List<string>? PartUrls { get; set; }
 }
 
 public sealed class ExportDetailResponse