fix: set disk cache max size to 4x configured blob size instead of hardcoded 10 GiB

The disk blob cache now uses 4 * BlobSizeLimit from config instead of a
hardcoded 10 GiB default. This ensures the cache scales with the
configured blob size.

internal/vaultik/blobcache.go

@@ -7,9 +7,6 @@ import (
 	"sync"
 )
 
-// defaultMaxBlobCacheBytes is the default maximum size of the disk blob cache (10 GB).
-const defaultMaxBlobCacheBytes = 10 << 30 // 10 GiB
-
 // blobDiskCacheEntry tracks a cached blob on disk.
 type blobDiskCacheEntry struct {
 	key  string

internal/vaultik/restore.go

@@ -109,7 +109,7 @@ func (v *Vaultik) Restore(opts *RestoreOptions) error {
 
 	// Step 5: Restore files
 	result := &RestoreResult{}
-	blobCache, err := newBlobDiskCache(defaultMaxBlobCacheBytes)
+	blobCache, err := newBlobDiskCache(4 * v.Config.BlobSizeLimit.Int64())
 	if err != nil {
 		return fmt.Errorf("creating blob cache: %w", err)
 	}