Integrate afero filesystem abstraction library

- Add afero.Fs field to Vaultik struct for filesystem operations
- Vaultik now owns and manages the filesystem instance
- SnapshotManager receives filesystem via SetFilesystem() setter
- Update blob packer to use afero for temporary files
- Convert all filesystem operations to use afero abstraction
- Remove filesystem module - Vaultik manages filesystem directly
- Update tests: remove symlink test (unsupported by afero memfs)
- Fix TestMultipleFileChanges to handle scanner examining directories

This enables full end-to-end testing without touching disk by using
memory-backed filesystems. Database operations continue using real
filesystem as SQLite requires actual files.

internal/snapshot/module.go

@@ -11,6 +11,7 @@ import (
 // ScannerParams holds parameters for scanner creation
 type ScannerParams struct {
 	EnableProgress bool
+	Fs             afero.Fs
 }
 
 // Module exports backup functionality as an fx module.
@@ -29,7 +30,7 @@ type ScannerFactory func(params ScannerParams) *Scanner
 func provideScannerFactory(cfg *config.Config, repos *database.Repositories, s3Client *s3.Client) ScannerFactory {
 	return func(params ScannerParams) *Scanner {
 		return NewScanner(ScannerConfig{
-			FS:               afero.NewOsFs(),
+			FS:               params.Fs,
 			ChunkSize:        cfg.ChunkSize.Int64(),
 			Repositories:     repos,
 			S3Client:         s3Client,