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/cli/app.go

@@ -9,7 +9,6 @@ import (
 	"time"
 
 	"git.eeqj.de/sneak/vaultik/internal/config"
-	"git.eeqj.de/sneak/vaultik/internal/crypto"
 	"git.eeqj.de/sneak/vaultik/internal/database"
 	"git.eeqj.de/sneak/vaultik/internal/globals"
 	"git.eeqj.de/sneak/vaultik/internal/log"
@@ -54,7 +53,6 @@ func NewApp(opts AppOptions) *fx.App {
 		log.Module,
 		s3.Module,
 		snapshot.Module,
-		crypto.Module, // This will provide crypto only if age_secret_key is configured
 		fx.Provide(vaultik.New),
 		fx.Invoke(setupGlobals),
 		fx.NopLogger,