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.

internal/server/server.go

@@ -29,15 +29,16 @@ type ScanTrigger interface {
 }
 
 type Server struct {
-	Logger      *slog.Logger
-	Pool        *pgxpool.Pool
-	Scanner     ScanTrigger
-	SubsonicCfg subsonic.Config
-	EventsCfg   config.EventsConfig
+	Logger            *slog.Logger
+	Pool              *pgxpool.Pool
+	Scanner           ScanTrigger
+	SubsonicCfg       subsonic.Config
+	EventsCfg         config.EventsConfig
+	RecommendationCfg config.RecommendationConfig
 }
 
-func New(logger *slog.Logger, pool *pgxpool.Pool, scanner ScanTrigger, subCfg subsonic.Config, eventsCfg config.EventsConfig) *Server {
-	return &Server{Logger: logger, Pool: pool, Scanner: scanner, SubsonicCfg: subCfg, EventsCfg: eventsCfg}
+func New(logger *slog.Logger, pool *pgxpool.Pool, scanner ScanTrigger, subCfg subsonic.Config, eventsCfg config.EventsConfig, recCfg config.RecommendationConfig) *Server {
+	return &Server{Logger: logger, Pool: pool, Scanner: scanner, SubsonicCfg: subCfg, EventsCfg: eventsCfg, RecommendationCfg: recCfg}
 }
 
 func (s *Server) Router() http.Handler {
@@ -54,7 +55,7 @@ func (s *Server) Router() http.Handler {
 			s.EventsCfg.SkipMaxCompletionRatio,
 			s.EventsCfg.SkipMaxDurationPlayedMs,
 		)
-		api.Mount(r, s.Pool, s.Logger, writer)
+		api.Mount(r, s.Pool, s.Logger, writer, s.RecommendationCfg)
 		r.Route("/api/admin", func(admin chi.Router) {
 			admin.Use(auth.RequireAdmin(s.Pool))
 			if s.Scanner != nil {