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/storage/url.go

@@ -8,18 +8,20 @@ import (
 
 // StorageURL represents a parsed storage URL.
 type StorageURL struct {
-	Scheme   string // "s3" or "file"
-	Bucket   string // S3 bucket name (empty for file)
-	Prefix   string // Path within bucket or filesystem base path
-	Endpoint string // S3 endpoint (optional, default AWS)
-	Region   string // S3 region (optional)
-	UseSSL   bool   // Use HTTPS for S3 (default true)
+	Scheme       string // "s3", "file", or "rclone"
+	Bucket       string // S3 bucket name (empty for file/rclone)
+	Prefix       string // Path within bucket or filesystem base path
+	Endpoint     string // S3 endpoint (optional, default AWS)
+	Region       string // S3 region (optional)
+	UseSSL       bool   // Use HTTPS for S3 (default true)
+	RcloneRemote string // rclone remote name (for rclone:// URLs)
 }
 
 // ParseStorageURL parses a storage URL string.
 // Supported formats:
 //   - s3://bucket/prefix?endpoint=host&region=us-east-1&ssl=true
 //   - file:///absolute/path/to/backup
+//   - rclone://remote/path/to/backups
 func ParseStorageURL(rawURL string) (*StorageURL, error) {
 	if rawURL == "" {
 		return nil, fmt.Errorf("storage URL is empty")
@@ -67,7 +69,28 @@ func ParseStorageURL(rawURL string) (*StorageURL, error) {
 		}, nil
 	}
 
-	return nil, fmt.Errorf("unsupported URL scheme: must start with s3:// or file://")
+	// Handle rclone:// URLs
+	if strings.HasPrefix(rawURL, "rclone://") {
+		u, err := url.Parse(rawURL)
+		if err != nil {
+			return nil, fmt.Errorf("invalid URL: %w", err)
+		}
+
+		remote := u.Host
+		if remote == "" {
+			return nil, fmt.Errorf("rclone URL missing remote name")
+		}
+
+		path := strings.TrimPrefix(u.Path, "/")
+
+		return &StorageURL{
+			Scheme:       "rclone",
+			Prefix:       path,
+			RcloneRemote: remote,
+		}, nil
+	}
+
+	return nil, fmt.Errorf("unsupported URL scheme: must start with s3://, file://, or rclone://")
 }
 
 // String returns a human-readable representation of the storage URL.
@@ -84,6 +107,11 @@ func (u *StorageURL) String() string {
 			return fmt.Sprintf("s3://%s/%s (endpoint: %s)", u.Bucket, u.Prefix, endpoint)
 		}
 		return fmt.Sprintf("s3://%s (endpoint: %s)", u.Bucket, endpoint)
+	case "rclone":
+		if u.Prefix != "" {
+			return fmt.Sprintf("rclone://%s/%s", u.RcloneRemote, u.Prefix)
+		}
+		return fmt.Sprintf("rclone://%s", u.RcloneRemote)
 	default:
 		return fmt.Sprintf("%s://?", u.Scheme)
 	}