BlobUploadFinalizeRequest

Declare a segmented upload complete — POST /api/blobs/uploads/{id}/finalize. The
expected values are CONCURRENCY tokens ("nothing landed since my last append"): both
are server-handed in [BlobUploadProgress], echoed back verbatim, never parsed. A
mismatch refuses with the staging kept, resumable — the ingest precedent's posture.

{
    "expected_content_hash": "string",
    "expected_segments": 0,
    "expected_total_bytes": 0
}