feat(api): add /api/radio stub returning the seed track only

M6 stub. Validates seed_track param (400 on missing/blank/bad UUID),
looks up the track (404 on miss), returns RadioResponse with a single
TrackRef. M4 will replace the body with similarity-driven candidate
pool + scoring; the request/response shape is final.

internal/api/radio_test.go

@@ -0,0 +1,90 @@
+package api
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+)
+
+func newRadioRouter(h *handlers) chi.Router {
+	r := chi.NewRouter()
+	r.Get("/api/radio", h.handleRadio)
+	return r
+}
+
+func TestHandleRadio_Stub_ReturnsSeedOnly(t *testing.T) {
+	h, pool := testHandlers(t)
+	truncateLibrary(t, pool)
+	artist := seedArtist(t, pool, "Beatles")
+	album := seedAlbum(t, pool, artist.ID, "Abbey Road", 1969)
+	track := seedTrack(t, pool, album.ID, artist.ID, "Something", 3, 183_000)
+
+	req := httptest.NewRequest(http.MethodGet, "/api/radio?seed_track="+uuidToString(track.ID), nil)
+	w := httptest.NewRecorder()
+	newRadioRouter(h).ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("status = %d body=%s", w.Code, w.Body.String())
+	}
+	var got struct {
+		Tracks []TrackRef `json:"tracks"`
+	}
+	if err := json.Unmarshal(w.Body.Bytes(), &got); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	if len(got.Tracks) != 1 {
+		t.Fatalf("len=%d, want 1", len(got.Tracks))
+	}
+	if got.Tracks[0].Title != "Something" {
+		t.Errorf("title=%q", got.Tracks[0].Title)
+	}
+	if got.Tracks[0].AlbumTitle != "Abbey Road" || got.Tracks[0].ArtistName != "Beatles" {
+		t.Errorf("ref = %+v", got.Tracks[0])
+	}
+}
+
+func TestHandleRadio_NotFound(t *testing.T) {
+	h, pool := testHandlers(t)
+	truncateLibrary(t, pool)
+
+	req := httptest.NewRequest(http.MethodGet, "/api/radio?seed_track=00000000-0000-0000-0000-000000000000", nil)
+	w := httptest.NewRecorder()
+	newRadioRouter(h).ServeHTTP(w, req)
+
+	if w.Code != http.StatusNotFound {
+		t.Fatalf("status = %d", w.Code)
+	}
+}
+
+func TestHandleRadio_MissingSeed(t *testing.T) {
+	h, _ := testHandlers(t)
+	req := httptest.NewRequest(http.MethodGet, "/api/radio", nil)
+	w := httptest.NewRecorder()
+	newRadioRouter(h).ServeHTTP(w, req)
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("status=%d, want 400", w.Code)
+	}
+}
+
+func TestHandleRadio_BlankSeed(t *testing.T) {
+	h, _ := testHandlers(t)
+	req := httptest.NewRequest(http.MethodGet, "/api/radio?seed_track=%20%20", nil)
+	w := httptest.NewRecorder()
+	newRadioRouter(h).ServeHTTP(w, req)
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("status=%d, want 400", w.Code)
+	}
+}
+
+func TestHandleRadio_BadUUID(t *testing.T) {
+	h, _ := testHandlers(t)
+	req := httptest.NewRequest(http.MethodGet, "/api/radio?seed_track=not-a-uuid", nil)
+	w := httptest.NewRecorder()
+	newRadioRouter(h).ServeHTTP(w, req)
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("status=%d, want 400", w.Code)
+	}
+}