Upload Asset

Bring-your-own asset upload — the FAST path for an agent that built a graphic in its own sandbox. The agent curls the raw bytes here over HTTP (multipart) instead of re-typing ~63K base64 chars through the model; the response returns a short-lived, user-bound upload_ref to pass to niche_attach_image. Stores to the owner-only R2 bucket; the ref maps to it in Redis. Validates type/size/decodability up front with named errors.