feat: taste onboarding for new listeners (M6)

- GET /onboarding offers the library's top artists (with album covers)
  and genres and says whether to show it (not done and < 10 plays);
  PUT /me/onboarding stores the choice or a skip (migration 00009).
- The recommender seeds the taste from it while the user has fewer than
  30 scored tracks: chosen artists' tracks +1, chosen genres +0.5.
- Web: onboarding replaces the wave on the home page until done or
  skipped; artist grid with covers, genre chips, at least 3 artists.
- e2e journey and the recommender DB test cover it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/api/cmd/api/e2e_test.go

@@ -271,6 +271,19 @@ func TestUserJourney(t *testing.T) {
 	c.ok("GET", "/api/v1/me/likes", nil, &likes)
 	require.Equal(t, 1, likes.Total)
 
+	// 7½. Онбординг: варианты из библиотеки, выбор сохраняется, второй раз не предлагается.
+	var onb struct {
+		Required bool `json:"required"`
+		Artists  []struct {
+			ID string `json:"id"`
+		} `json:"artists"`
+	}
+	c.ok("GET", "/api/v1/onboarding", nil, &onb)
+	require.NotEmpty(t, onb.Artists)
+	c.ok("PUT", "/api/v1/me/onboarding", map[string]any{"artist_ids": []string{onb.Artists[0].ID}, "genres": []string{}}, nil)
+	c.ok("GET", "/api/v1/onboarding", nil, &onb)
+	require.False(t, onb.Required)
+
 	// 8. «Моя волна»: рекомендатель недоступен — запасная волна из библиотеки.
 	var wave struct {
 		SessionID string  `json:"session_id"`