Add UploadAttachment, a unary RPC that accepts a file in bounded chunks
and streams it to a temp file instead of holding the whole blob in one
request body and in memory. A call carrying a spec starts an upload and
returns an opaque upload_id; later calls carry that id with
write_offset,
data, and finish_write, following the Cloud Storage WriteObject shape.
Retrying the last chunk after a lost response is idempotent, uploads are
bound to their owner, and they expire after 30 minutes of inactivity.
CreateAttachment and the chunked finalize now share one processing
pipeline for motion-photo detection, EXIF stripping, and storage. The
motion-photo detector works over an io.ReaderAt so large JPEGs are never
loaded whole, and the request body cap is a single per-procedure lookup.
The web editor uploads through the new RPC in 2 MiB chunks with bounded
concurrency under the server's per-user limit.
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