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>

lib/render-pipeline/artifact-kinds.ts

@@ -0,0 +1,18 @@
+/**
+ * Worker artifact kinds (E20) and the object-storage folder each lands in:
+ * preview → videos/, thumbnail → image/, metadata/log → renders/, highres (the
+ * delivery MOV) → highres/ beside the manually uploaded High Res files.
+ */
+export const ARTIFACT_KIND_FOLDERS: Record<string, string> = {
+  preview: "videos",
+  thumbnail: "image",
+  metadata: "renders",
+  log: "renders",
+  highres: "highres",
+};
+
+/** True for keys artifact-presign could have issued — guards E20b. */
+export function isArtifactKey(key: string): boolean {
+  const folder = key.split("/")[0];
+  return key.includes("/") && !key.includes("..") && Object.values(ARTIFACT_KIND_FOLDERS).includes(folder);
+}