datannurpy v0.23.0

• add: export dataset previews as JSON and JSON-JS files, controlled by global and per-source preview_rows
• add: datasets now expose has_preview when preview files are available
• fix: metadata dataset._match_path now strictly overrides data_path for scan matching, allowing exported data_path to remain a public URL
• fix: local file scans now export portable data_path values while keeping absolute _match_path values for runtime matching