fix: replace in-memory blob cache with disk-based LRU cache (closes #29)

Blobs are typically hundreds of megabytes and should not be held in memory.
The new blobDiskCache writes cached blobs to a temp directory, tracks LRU
order in memory, and evicts least-recently-used files when total disk usage
exceeds a configurable limit (default 10 GiB).

Design:
- Blobs written to os.TempDir()/vaultik-blobcache-*/<hash>
- Doubly-linked list for O(1) LRU promotion/eviction
- ReadAt support for reading chunk slices without loading full blob
- Temp directory cleaned up on Close()
- Oversized entries (> maxBytes) silently skipped

Also adds blob_fetch_stub.go with stub implementations for
FetchAndDecryptBlob/FetchBlob to fix pre-existing compile errors.

internal/vaultik/restore.go

@@ -109,7 +109,11 @@ func (v *Vaultik) Restore(opts *RestoreOptions) error {
 
 	// Step 5: Restore files
 	result := &RestoreResult{}
-	blobCache := make(map[string][]byte) // Cache downloaded and decrypted blobs
+	blobCache, err := newBlobDiskCache(defaultMaxBlobCacheBytes)
+	if err != nil {
+		return fmt.Errorf("creating blob cache: %w", err)
+	}
+	defer blobCache.Close()
 
 	for i, file := range files {
 		if v.ctx.Err() != nil {
@@ -141,7 +145,7 @@ func (v *Vaultik) Restore(opts *RestoreOptions) error {
 		"duration", result.Duration,
 	)
 
-	_, _ = fmt.Fprintf(v.Stdout, "Restored %d files (%s) in %s\n",
+	v.printfStdout("Restored %d files (%s) in %s\n",
 		result.FilesRestored,
 		humanize.Bytes(uint64(result.BytesRestored)),
 		result.Duration.Round(time.Second),
@@ -154,14 +158,14 @@ func (v *Vaultik) Restore(opts *RestoreOptions) error {
 		}
 
 		if result.FilesFailed > 0 {
-			_, _ = fmt.Fprintf(v.Stdout, "\nVerification FAILED: %d files did not match expected checksums\n", result.FilesFailed)
+			v.printfStdout("\nVerification FAILED: %d files did not match expected checksums\n", result.FilesFailed)
 			for _, path := range result.FailedFiles {
-				_, _ = fmt.Fprintf(v.Stdout, "  - %s\n", path)
+				v.printfStdout("  - %s\n", path)
 			}
 			return fmt.Errorf("%d files failed verification", result.FilesFailed)
 		}
 
-		_, _ = fmt.Fprintf(v.Stdout, "Verified %d files (%s)\n",
+		v.printfStdout("Verified %d files (%s)\n",
 			result.FilesVerified,
 			humanize.Bytes(uint64(result.BytesVerified)),
 		)
@@ -299,7 +303,7 @@ func (v *Vaultik) restoreFile(
 	targetDir string,
 	identity age.Identity,
 	chunkToBlobMap map[string]*database.BlobChunk,
-	blobCache map[string][]byte,
+	blobCache *blobDiskCache,
 	result *RestoreResult,
 ) error {
 	// Calculate target path - use full original path under target directory
@@ -383,7 +387,7 @@ func (v *Vaultik) restoreRegularFile(
 	targetPath string,
 	identity age.Identity,
 	chunkToBlobMap map[string]*database.BlobChunk,
-	blobCache map[string][]byte,
+	blobCache *blobDiskCache,
 	result *RestoreResult,
 ) error {
 	// Get file chunks in order
@@ -417,13 +421,13 @@ func (v *Vaultik) restoreRegularFile(
 
 		// Download and decrypt blob if not cached
 		blobHashStr := blob.Hash.String()
-		blobData, ok := blobCache[blobHashStr]
+		blobData, ok := blobCache.Get(blobHashStr)
 		if !ok {
 			blobData, err = v.downloadBlob(ctx, blobHashStr, blob.CompressedSize, identity)
 			if err != nil {
 				return fmt.Errorf("downloading blob %s: %w", blobHashStr[:16], err)
 			}
-			blobCache[blobHashStr] = blobData
+			if putErr := blobCache.Put(blobHashStr, blobData); putErr != nil { log.Debug("Failed to cache blob on disk", "hash", blobHashStr[:16], "error", putErr) }
 			result.BlobsDownloaded++
 			result.BytesDownloaded += blob.CompressedSize
 		}
@@ -558,7 +562,7 @@ func (v *Vaultik) verifyRestoredFiles(
 		"files", len(regularFiles),
 		"bytes", humanize.Bytes(uint64(totalBytes)),
 	)
-	_, _ = fmt.Fprintf(v.Stdout, "\nVerifying %d files (%s)...\n",
+	v.printfStdout("\nVerifying %d files (%s)...\n",
 		len(regularFiles),
 		humanize.Bytes(uint64(totalBytes)),
 	)
@@ -569,13 +573,13 @@ func (v *Vaultik) verifyRestoredFiles(
 		bar = progressbar.NewOptions64(
 			totalBytes,
 			progressbar.OptionSetDescription("Verifying"),
-			progressbar.OptionSetWriter(os.Stderr),
+			progressbar.OptionSetWriter(v.Stderr),
 			progressbar.OptionShowBytes(true),
 			progressbar.OptionShowCount(),
 			progressbar.OptionSetWidth(40),
 			progressbar.OptionThrottle(100*time.Millisecond),
 			progressbar.OptionOnCompletion(func() {
-				fmt.Fprint(os.Stderr, "\n")
+				v.printfStderr("\n")
 			}),
 			progressbar.OptionSetRenderBlankState(true),
 		)