The S3 upload path used temp files (vikunja-s3-upload-*) to buffer
non-seekable readers. In Docker containers with restrictive permissions,
these temp files could not be created, causing "permission denied"
errors for avatar and background image uploads.
By changing the file storage API (Create, CreateWithMime,
CreateWithMimeAndSession, Save) to require io.ReadSeeker instead of
io.Reader, the temp file fallback is no longer needed and is removed.
This enforces at the type level that all callers provide seekable
readers, preventing this class of bug from recurring.
Closesgo-vikunja/vikunja#2185