feat(api): radio handler reads current session vector + threads ContextWeight

Wire contextual scoring end-to-end: add ContextWeight to RecommendationConfig
(struct + Default()), fetch the user's current session vector in handleRadio
via loadCurrentSessionVector (cold-start returns Seed sentinel), and pass it
as the 6th arg to LoadCandidates so ContextualMatchScore flows into Shuffle.

internal/api/auth_test.go

@@ -52,7 +52,7 @@ func testHandlers(t *testing.T) (*handlers, *pgxpool.Pool) {
 	w := playevents.NewWriter(pool, logger, 30*time.Minute, 0.5, 30000)
 	recCfg := config.RecommendationConfig{
 		BaseWeight: 1.0, LikeBoost: 2.0, RecencyWeight: 1.0,
-		SkipPenalty: 1.0, JitterMagnitude: 0.1,
+		SkipPenalty: 1.0, JitterMagnitude: 0.1, ContextWeight: 2.0,
 		RecentlyPlayedHours: 1, RadioSize: 50, RadioSizeMax: 200,
 	}
 	h := &handlers{pool: pool, logger: logger, events: w, recCfg: recCfg, rng: func() float64 { return 0.5 }}