feat(api): radio uses similarity pool with exclude param + M3 fallback

Wire LoadCandidatesFromSimilarity as the primary candidate loader with
an ?exclude= query param for comma-separated UUID filtering; fall back
to LoadCandidates on error. Thread SimilarityWeight into ScoringWeights
and update testHandlers recCfg accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/api/auth_test.go

@@ -52,7 +52,8 @@ 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, ContextWeight: 2.0,
+		SkipPenalty: 1.0, JitterMagnitude: 0.1,
+		ContextWeight: 2.0, SimilarityWeight: 2.0,
 		RecentlyPlayedHours: 1, RadioSize: 50, RadioSizeMax: 200,
 	}
 	h := &handlers{pool: pool, logger: logger, events: w, recCfg: recCfg, rng: func() float64 { return 0.5 }}