Create qbz-library crate with:
- LibraryScanner: directory scanning for audio files
- MetadataExtractor: audio metadata extraction via lofty
- LibraryDatabase: SQLite persistence with WAL mode
- CueParser: CUE sheet parsing for single-file albums
- Thumbnails: artwork extraction and thumbnail generation
- Tag sidecar: album metadata override files
src-tauri re-exports from qbz-library for backwards compatibility.
Enables TUI/headless clients to manage local libraries without Tauri.
