Replace the third-party afero-s3 library (and its temporary fork) with
a minimal in-tree afero.Fs implementation (~200 lines) that only supports
the three S3 operations Vikunja actually uses: Open (GetObject), Remove
(DeleteObject), and Stat (HeadObject).
The s3File implementation lazily opens a GetObject stream on first Read
and supports Seek by closing and re-opening the stream from the new offset,
matching the behavior of the fixed afero-s3 fork.
All other afero.Fs/File methods return ErrS3NotSupported since they are
never called in the S3 code path (writes already use direct PutObject).
This removes the fclairamb/afero-s3 dependency and the temporary replace
directive in go.mod entirely.
Closes#2347
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
b065c62007