## Background
`MinioStorage.Save` is called with `size = -1` on several paths,
including Actions logs, Actions artifacts, repository archives, avatars
and attachments. With an unknown size (-1) minio-go assumes a 5TiB
object and allocates a single part-sized buffer of 528MiB per upload,
regardless of the real payload size, which can exhaust the memory of
small instances.
Measured against a local S3 stub, five sequential uploads of a 4KiB
payload grew RSS by 1041MiB before the fix and by 34MiB after it.
## Fix
Pass an explicit 16MiB part size in that case, the same value minio-go
uses as minimum part size
(https://github.com/minio/minio-go/blob/v7.2.1/constants.go#L28).
Uploads with a known size are left untouched, since minio-go already
derives a part size proportional to the real object size.
## Note:
One behaviour change: with an unknown size the object is now limited to
16MiB * 10000 parts = 156.25GiB
(https://github.com/minio/minio-go/blob/v7.2.1/api-put-object-common.go#L112-L116).
`putObjectMultipartStreamNoLength` completes the upload once it runs out
of parts without checking that the reader was drained, so a stream past
that limit is silently truncated rather than rejected. The previous
limit was 5TiB. No payload Gitea uploads comes close to either.
Parts are also uploaded serially, so a smaller part size means
proportionally more round trips for large unknown-size uploads.
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Co-authored-by: silverwind <me@silverwind.io>
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