BlobUploadBeginRequest

Begin a segmented upload — POST /api/blobs/uploads. Declares the whole upload up
front: the home the assembled blob will commit into, and the media type it will
commit under. Both are re-examined at finalize — standing by the service (it can
change mid-upload), the allowlist by the SQL wrapper (the sole authority, D9).

{
    "content_type": "string",
    "home_id": "62b88c0b-1769-4935-8269-4fff8db117a7",
    "home_table": "string"
}