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.
Track old-to-new attachment ID mapping during duplication and
re-set CoverImageAttachmentID on the new task if the original
had a cover image configured.
Save the source file handle before calling NewAttachment (which
overwrites attachment.File) and use defer to ensure it gets closed.
This prevents file descriptor leaks on both success and error paths.
