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/api/api.go

@@ -6,19 +6,22 @@ package api
 
 import (
 	"log/slog"
+	"math/rand"
 
 	"github.com/go-chi/chi/v5"
 	"github.com/jackc/pgx/v5/pgxpool"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/auth"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/config"
 	"git.fabledsword.com/bvandeusen/minstrel/internal/playevents"
 )
 
 // Mount attaches /api/* handlers to r. Public endpoints (login) are outside
 // RequireUser; everything else is gated by the middleware. The events writer
 // is shared with the Subsonic mount so /rest/scrobble feeds the same store.
-func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer) {
-	h := &handlers{pool: pool, logger: logger, events: events}
+func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playevents.Writer, recCfg config.RecommendationConfig) {
+	rng := rand.New(rand.NewSource(rand.Int63()))
+	h := &handlers{pool: pool, logger: logger, events: events, recCfg: recCfg, rng: rng.Float64}
 
 	r.Route("/api", func(api chi.Router) {
 		api.Post("/auth/login", h.handleLogin)
@@ -54,4 +57,6 @@ type handlers struct {
 	pool   *pgxpool.Pool
 	logger *slog.Logger
 	events *playevents.Writer
+	recCfg config.RecommendationConfig
+	rng    func() float64
 }