Finish a multipart upload for a scratch object.

Same body as `POST /v1/assets/complete-upload`. Commits the multipart upload, records the final object size against the workspace's storage usage, and moves the scratch object to `active`. Only the user who created the scratch object may call this (403 otherwise). Idempotent: calling it on an already-`active` object returns the same `{ id, url }` without changing anything.

curl -X POST "https://example.com/scratch/497f6eca-6276-4993-bfeb-53cbbbba6f08/complete-upload" \  -H "Content-Type: application/json" \  -d '{    "uploadId": "string",    "parts": [      {        "ETag": "string",        "PartNumber": 1      }    ]  }'

{  "id": "497f6eca-6276-4993-bfeb-53cbbbba6f08",  "url": "string"}