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/blob/packer.go

@@ -20,7 +20,6 @@ import (
 	"encoding/hex"
 	"fmt"
 	"io"
-	"os"
 	"sync"
 	"time"
 
@@ -28,6 +27,7 @@ import (
 	"git.eeqj.de/sneak/vaultik/internal/database"
 	"git.eeqj.de/sneak/vaultik/internal/log"
 	"github.com/google/uuid"
+	"github.com/spf13/afero"
 )
 
 // BlobHandler is a callback function invoked when a blob is finalized and ready for upload.
@@ -44,6 +44,7 @@ type PackerConfig struct {
 	Recipients       []string               // Age recipients for encryption
 	Repositories     *database.Repositories // Database repositories for tracking blob metadata
 	BlobHandler      BlobHandler            // Optional callback when blob is ready for upload
+	Fs               afero.Fs               // Filesystem for temporary files
 }
 
 // Packer accumulates chunks and packs them into blobs.
@@ -55,6 +56,7 @@ type Packer struct {
 	recipients       []string               // Age recipients for encryption
 	blobHandler      BlobHandler            // Called when blob is ready
 	repos            *database.Repositories // For creating blob records
+	fs               afero.Fs               // Filesystem for temporary files
 
 	// Mutex for thread-safe blob creation
 	mu sync.Mutex
@@ -69,7 +71,7 @@ type blobInProgress struct {
 	id        string          // UUID of the blob
 	chunks    []*chunkInfo    // Track chunk metadata
 	chunkSet  map[string]bool // Track unique chunks in this blob
-	tempFile  *os.File        // Temporary file for encrypted compressed data
+	tempFile  afero.File      // Temporary file for encrypted compressed data
 	writer    *blobgen.Writer // Unified compression/encryption/hashing writer
 	startTime time.Time
 	size      int64 // Current uncompressed size
@@ -113,7 +115,7 @@ type BlobChunkRef struct {
 type BlobWithReader struct {
 	*FinishedBlob
 	Reader   io.ReadSeeker
-	TempFile *os.File // Optional, only set for disk-based blobs
+	TempFile afero.File // Optional, only set for disk-based blobs
 }
 
 // NewPacker creates a new blob packer that accumulates chunks into blobs.
@@ -126,12 +128,16 @@ func NewPacker(cfg PackerConfig) (*Packer, error) {
 	if cfg.MaxBlobSize <= 0 {
 		return nil, fmt.Errorf("max blob size must be positive")
 	}
+	if cfg.Fs == nil {
+		return nil, fmt.Errorf("filesystem is required")
+	}
 	return &Packer{
 		maxBlobSize:      cfg.MaxBlobSize,
 		compressionLevel: cfg.CompressionLevel,
 		recipients:       cfg.Recipients,
 		blobHandler:      cfg.BlobHandler,
 		repos:            cfg.Repositories,
+		fs:               cfg.Fs,
 		finishedBlobs:    make([]*FinishedBlob, 0),
 	}, nil
 }
@@ -255,7 +261,7 @@ func (p *Packer) startNewBlob() error {
 	}
 
 	// Create temporary file
-	tempFile, err := os.CreateTemp("", "vaultik-blob-*.tmp")
+	tempFile, err := afero.TempFile(p.fs, "", "vaultik-blob-*.tmp")
 	if err != nil {
 		return fmt.Errorf("creating temp file: %w", err)
 	}
@@ -264,7 +270,7 @@ func (p *Packer) startNewBlob() error {
 	writer, err := blobgen.NewWriter(tempFile, p.compressionLevel, p.recipients)
 	if err != nil {
 		_ = tempFile.Close()
-		_ = os.Remove(tempFile.Name())
+		_ = p.fs.Remove(tempFile.Name())
 		return fmt.Errorf("creating blobgen writer: %w", err)
 	}
 
@@ -469,7 +475,7 @@ func (p *Packer) cleanupTempFile() {
 	if p.currentBlob != nil && p.currentBlob.tempFile != nil {
 		name := p.currentBlob.tempFile.Name()
 		_ = p.currentBlob.tempFile.Close()
-		_ = os.Remove(name)
+		_ = p.fs.Remove(name)
 	}
 }