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/radio_test.go

@@ -131,6 +131,110 @@ func TestHandleRadio_LimitClampedToMax(t *testing.T) {
 	}
 }
 
+func TestHandleRadio_WithSimilarityPool_RanksLBSimilarHigher(t *testing.T) {
+	h, pool := testHandlers(t)
+	truncateLibrary(t, pool)
+	user := seedUser(t, pool, "alice", "x", false)
+	artist := seedArtist(t, pool, "X")
+	album := seedAlbum(t, pool, artist.ID, "X", 1990)
+	seed := seedTrack(t, pool, album.ID, artist.ID, "Seed", 1, 100_000)
+	target := seedTrack(t, pool, album.ID, artist.ID, "Target", 2, 100_000)
+	control := seedTrack(t, pool, album.ID, artist.ID, "Control", 3, 100_000)
+	if _, err := pool.Exec(context.Background(),
+		`INSERT INTO track_similarity (track_a_id, track_b_id, score, source) VALUES ($1, $2, 0.95, 'listenbrainz')`,
+		seed.ID, target.ID); err != nil {
+		t.Fatalf("insert sim: %v", err)
+	}
+	w := callRadio(h, user, "seed_track="+uuidToString(seed.ID)+"&limit=3")
+	if w.Code != http.StatusOK {
+		t.Fatalf("status = %d body=%s", w.Code, w.Body.String())
+	}
+	var resp RadioResponse
+	_ = json.Unmarshal(w.Body.Bytes(), &resp)
+	targetIdx, controlIdx := -1, -1
+	for i, tr := range resp.Tracks {
+		if tr.ID == uuidToString(target.ID) {
+			targetIdx = i
+		}
+		if tr.ID == uuidToString(control.ID) {
+			controlIdx = i
+		}
+	}
+	if targetIdx < 0 || controlIdx < 0 {
+		t.Fatalf("target=%d control=%d (one not present)", targetIdx, controlIdx)
+	}
+	if targetIdx >= controlIdx {
+		t.Errorf("target ranked at %d, control at %d — LB-similar should rank higher", targetIdx, controlIdx)
+	}
+}
+
+func TestHandleRadio_ExcludeParamFiltersOut(t *testing.T) {
+	h, pool := testHandlers(t)
+	truncateLibrary(t, pool)
+	user := seedUser(t, pool, "alice", "x", false)
+	artist := seedArtist(t, pool, "X")
+	album := seedAlbum(t, pool, artist.ID, "X", 1990)
+	seed := seedTrack(t, pool, album.ID, artist.ID, "Seed", 1, 100_000)
+	excluded := seedTrack(t, pool, album.ID, artist.ID, "Excluded", 2, 100_000)
+	for i := 3; i <= 6; i++ {
+		seedTrack(t, pool, album.ID, artist.ID, "T"+string(rune('0'+i)), i, 100_000)
+	}
+	q := "seed_track=" + uuidToString(seed.ID) + "&limit=10&exclude=" + uuidToString(excluded.ID)
+	w := callRadio(h, user, q)
+	if w.Code != http.StatusOK {
+		t.Fatalf("status = %d", w.Code)
+	}
+	var resp RadioResponse
+	_ = json.Unmarshal(w.Body.Bytes(), &resp)
+	for _, tr := range resp.Tracks {
+		if tr.ID == uuidToString(excluded.ID) {
+			t.Error("excluded track present in response")
+		}
+	}
+}
+
+func TestHandleRadio_ExcludeParamMalformedSkipped(t *testing.T) {
+	h, pool := testHandlers(t)
+	truncateLibrary(t, pool)
+	user := seedUser(t, pool, "alice", "x", false)
+	artist := seedArtist(t, pool, "X")
+	album := seedAlbum(t, pool, artist.ID, "X", 1990)
+	seed := seedTrack(t, pool, album.ID, artist.ID, "Seed", 1, 100_000)
+	other := seedTrack(t, pool, album.ID, artist.ID, "Other", 2, 100_000)
+	q := "seed_track=" + uuidToString(seed.ID) + "&limit=5&exclude=not-a-uuid," + uuidToString(other.ID)
+	w := callRadio(h, user, q)
+	if w.Code != http.StatusOK {
+		t.Fatalf("status = %d (malformed UUID should be silently dropped)", w.Code)
+	}
+	var resp RadioResponse
+	_ = json.Unmarshal(w.Body.Bytes(), &resp)
+	for _, tr := range resp.Tracks {
+		if tr.ID == uuidToString(other.ID) {
+			t.Error("other track should still be excluded after dropping malformed entry")
+		}
+	}
+}
+
+func TestHandleRadio_SeedAlwaysAtIndex0(t *testing.T) {
+	// Defensive: even when the similarity pool returns no candidates,
+	// the seed track must still be the first track in the response.
+	h, pool := testHandlers(t)
+	truncateLibrary(t, pool)
+	user := seedUser(t, pool, "alice", "x", false)
+	artist := seedArtist(t, pool, "X")
+	album := seedAlbum(t, pool, artist.ID, "X", 1990)
+	seed := seedTrack(t, pool, album.ID, artist.ID, "Seed", 1, 100_000)
+	w := callRadio(h, user, "seed_track="+uuidToString(seed.ID))
+	if w.Code != http.StatusOK {
+		t.Fatalf("status = %d", w.Code)
+	}
+	var resp RadioResponse
+	_ = json.Unmarshal(w.Body.Bytes(), &resp)
+	if len(resp.Tracks) == 0 || resp.Tracks[0].ID != uuidToString(seed.ID) {
+		t.Errorf("seed not at index 0: %v", resp.Tracks)
+	}
+}
+
 func TestHandleRadio_ContextualMatch_BoostsRankingOverControl(t *testing.T) {
 	h, pool := testHandlers(t)
 	truncateLibrary(t, pool)