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/migrations/00009_onboarding.sql

@@ -0,0 +1,7 @@
+-- +goose Up
+-- Онбординг «Моей волны» (docs/recommendations.md#7): выбранные исполнители и жанры — начальный вкус,
+-- пока своей истории мало. NULL — онбординг ещё не пройден и не пропущен.
+ALTER TABLE user_profiles_rec ADD COLUMN onboarding jsonb;
+
+-- +goose Down
+ALTER TABLE user_profiles_rec DROP COLUMN onboarding;