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/config/config.go

@@ -297,7 +297,11 @@ func (c *Config) validateStorage() error {
 			}
 			return nil
 		}
-		return fmt.Errorf("storage_url must start with s3:// or file://")
+		if strings.HasPrefix(c.StorageURL, "rclone://") {
+			// Rclone storage uses rclone's own config
+			return nil
+		}
+		return fmt.Errorf("storage_url must start with s3://, file://, or rclone://")
 	}
 
 	// Legacy S3 configuration