feat(qbz-radio): agnostic smart-radio pool builder crate

Ports the Tauri radio_engine into a standalone, frontend-agnostic
crate so any client (Slint, TUI, CLI) can build and play a smart
radio without Tauri.

  - New crate qbz-radio (builder + db + engine + tests), copied
    verbatim from src-tauri/radio_engine with the only change being
    the dependency rewrite: crate::api::{QobuzClient, Track,
    TracksContainer} -> qbz_qobuz::QobuzClient + qbz_models. Depends
    only on qbz-qobuz + qbz-models + rusqlite + dirs. All four engine
    tests pass.
  - QbzCore::create_smart_artist_radio builds the session pool + pulls
    up to 50 track ids + resolves them to full tracks. The radio DB's
    rusqlite Connection is !Send, so the build + pull run on
    spawn_blocking threads (the async builder driven via block_on),
    mirroring the Tauri command.
  - The artist "radio" action (Spotlight RADIO card + artist radio)
    now uses the smart builder (playback::play_smart_artist_radio);
    album radio (Radio Stations) keeps the Qobuz /radio/album
    endpoint, so both radio systems are in use. play_artist_radio
    (Qobuz /radio/artist) is retained as the simpler alternative.

cc

crates/Cargo.toml

@@ -7,6 +7,7 @@ members = [
     "qbz-audio",
     "qbz-player",
     "qbz-qobuz",
+    "qbz-radio",
     "qbz-core",
     "qbz-integrations",
     "qbz-cache",