Add deterministic deduplication, rclone backend, and database purge command

- Implement deterministic blob hashing using double SHA256 of uncompressed
  plaintext data, enabling deduplication even after local DB is cleared
- Add Stat() check before blob upload to skip existing blobs in storage
- Add rclone storage backend for additional remote storage options
- Add 'vaultik database purge' command to erase local state DB
- Add 'vaultik remote check' command to verify remote connectivity
- Show configured snapshots in 'vaultik snapshot list' output
- Skip macOS resource fork files (._*) when listing remote snapshots
- Use multi-threaded zstd compression (CPUs - 2 threads)
- Add writer tests for double hashing behavior

internal/snapshot/progress.go

@@ -39,7 +39,7 @@ type ProgressStats struct {
 	BlobsCreated     atomic.Int64
 	BlobsUploaded    atomic.Int64
 	BytesUploaded    atomic.Int64
-	UploadDurationMs atomic.Int64 // Total milliseconds spent uploading to S3
+	UploadDurationMs atomic.Int64 // Total milliseconds spent uploading
 	CurrentFile      atomic.Value // stores string
 	TotalSize        atomic.Int64 // Total size to process (set after scan phase)
 	TotalFiles       atomic.Int64 // Total files to process in phase 2
@@ -273,7 +273,7 @@ func (pr *ProgressReporter) printDetailedStatus() {
 		"created", blobsCreated,
 		"uploaded", blobsUploaded,
 		"pending", blobsCreated-blobsUploaded)
-	log.Info("Total uploaded to S3",
+	log.Info("Total uploaded to remote",
 		"uploaded", humanize.Bytes(uint64(bytesUploaded)),
 		"compression_ratio", formatRatio(bytesUploaded, bytesScanned))
 	if currentFile != "" {
@@ -336,7 +336,7 @@ func (pr *ProgressReporter) ReportUploadStart(blobHash string, size int64) {
 	pr.stats.CurrentUpload.Store(info)
 
 	// Log the start of upload
-	log.Info("Starting blob upload to S3",
+	log.Info("Starting blob upload",
 		"hash", blobHash[:8]+"...",
 		"size", humanize.Bytes(uint64(size)))
 }