feat(api): rewrite /api/radio with weighted shuffle v1

Validates seed_track + optional limit (default cfg.Recommendation.RadioSize,
clamped to RadioSizeMax). Calls recommendation.LoadCandidates +
recommendation.Shuffle. Prepends seed to result. Server seeds a
math/rand source at startup; handlers package threads that as a
func() float64 so tests inject deterministic RNGs.

Mount + server.New gain a RecommendationConfig parameter.

cmd/minstrel/main.go

@@ -75,7 +75,7 @@ func run() error {
 
 	srv := server.New(logger, pool, scanner, subsonic.Config{
 		AllowPlaintextPassword: cfg.Subsonic.AllowPlaintextPassword,
-	}, cfg.Events)
+	}, cfg.Events, cfg.Recommendation)
 	httpServer := &http.Server{
 		Addr:              cfg.Server.Address,
 		Handler:           srv.Router(),