Replace the github.com/spf13/afero dependency with a purpose-built
FileStorage interface (Open, Write, Stat, Remove, MkdirAll) with three
implementations: localStorage (with basePath), s3Storage (with key
prefix), and memStorage (for tests).
Each implementation owns its base path — callers pass only file IDs.
Delete s3fs.go, change File.File from afero.File to io.ReadCloser,
and fix duplication flows to buffer content for seeking.
