The `metadata` JSON part in the manga-create and chapter-upload handlers was
read with `Field::bytes()`, buffering the whole part before any check — a client
could send a multi-hundred-MB metadata blob (up to the 200 MiB request limit) as
one allocation. Image parts were already streamed with a cap; metadata was the
last unbounded read.
Cap it at a shared upload::MAX_METADATA_BYTES (64 KiB) via the read_capped
streaming loop (413 on overflow), and remove the now-unused read_field_bytes
helper so the unbounded path can't return.
Tests: manga + chapter oversized-metadata parts are rejected with 413.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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