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/module.go

@@ -73,6 +73,9 @@ func storerFromURL(rawURL string, cfg *config.Config) (Storer, error) {
 		}
 		return NewS3Storer(client), nil
 
+	case "rclone":
+		return NewRcloneStorer(context.Background(), parsed.RcloneRemote, parsed.Prefix)
+
 	default:
 		return nil, fmt.Errorf("unsupported storage scheme: %s", parsed.Scheme)
 	}