Implement FilesystemStore satisfying the FileStore interface for local
filesystem storage, replacing MinIO for simpler deployments.
- Atomic writes via temp file + os.Rename (no partial files)
- SHA-256 checksum computed on Put via io.MultiWriter
- Get/GetVersion return os.File (GetVersion ignores versionID)
- Delete is idempotent (no error if file missing)
- Copy uses same atomic write pattern
- PresignPut returns ErrPresignNotSupported
- Ping verifies root directory is writable
- Wire NewFilesystemStore in main.go backend switch
- 14 unit tests covering all methods including atomicity
Closes#127
Extract a FileStore interface from the concrete *storage.Storage MinIO
wrapper so the API layer is storage-backend agnostic.
- Define FileStore interface in internal/storage/interface.go
- Add Exists method to MinIO Storage (via StatObject)
- Add compile-time interface satisfaction check
- Change Server.storage and ServerState.storage to FileStore interface
- Update NewServer and NewServerState signatures
- Add Backend and FilesystemConfig fields to StorageConfig
- Add backend selection switch in main.go (minio/filesystem/unknown)
- Update config.example.yaml with backend field
The nil-interface pattern is preserved: when storage is unconfigured,
store remains a true nil FileStore (not a typed nil pointer), so all
existing if s.storage == nil checks continue to work correctly.
Closes#126
