feat(reco): port SuggestionsEngine (direct-weight ranking + quality gates)

T5: 1:1 port of the artist_vectors SuggestionsEngine into qbz-reco (~880 lines;
structure matches the Tauri source function-for-function). generate_suggestions:
ensure each seed vector (builder.ensure_vector), gate on a non-empty combined
playlist vector, rank candidates by DIRECT summed relationship weight via
store.get_all_related_artists (NOT cosine/find_nearest, D3), then pull + dedup +
exclude tracks from Qobuz with the validate_qobuz_artist + has_incompatible_genre
gates and optional reason strings. Keeps Tauri's Arc<Mutex>/RwLock ownership
(Send-safe). DTO reconciliation: crate::api -> qbz_qobuz::QobuzClient +
qbz_models::Track (4 Qobuz calls mapped to real signatures, field-by-field).
Adds rand (0.10, workspace) + qbz-models deps. 23 qbz-reco tests green.

cc

crates/Cargo.lock

@@ -5823,7 +5823,9 @@ version = "0.1.0"
 dependencies = [
  "log",
  "qbz-integrations",
+ "qbz-models",
  "qbz-qobuz",
+ "rand 0.10.1",
  "rusqlite",
  "serde",
  "tokio",