feat(qbzd): login subcommand, library scan, service files (68 endpoints)

qbzd login: interactive OAuth via system browser. Opens browser,
captures callback, exchanges code, saves token to keyring. Same
flow as desktop app. Daemon auto-logins on next start.

Library scan endpoint (POST /api/library/scan): discovers audio
files in registered folders. Full metadata extraction deferred
until MetadataExtractor moves to qbz-library crate.

Service files: systemd, OpenRC, runit + example qbzd.toml config.

cc

crates/qbzd/src/daemon.rs

@@ -413,6 +413,7 @@ fn build_router(daemon: Arc<DaemonCore>) -> axum::Router {
         .route("/api/library/folders", get(with_daemon!(daemon, library::get_folders)))
         .route("/api/library/folders", post(with_daemon!(daemon, library::add_folder, json)))
         .route("/api/library/folders", delete(with_daemon!(daemon, library::remove_folder, json)))
+        .route("/api/library/scan", post(with_daemon!(daemon, library::start_scan)))
         // Integrations
         .route("/api/integrations/listenbrainz", get(with_daemon!(daemon, integrations::get_listenbrainz_status)))
         .route("/api/integrations/listenbrainz/connect", post(with_daemon!(daemon, integrations::connect_listenbrainz, json)))