Upload Assembly File

Stream an assembly file to R2 server-side (firewall-safe upload proxy).

Replaces the browser's direct PUT to *.r2.cloudflarestorage.com. The client POSTs the file as multipart/form-data to this endpoint; the API streams it straight to R2. The client must still call POST /confirm-upload afterwards.

Authorization: the project must be owned by the current user, the project status must still allow mutations, and the assembly file row (created by /initiate-upload) must be in UPLOADING status. The file is written to the row's stored r2_key — the client cannot choose an arbitrary key.