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/recommender/tests/test_db_integration.py

@@ -3,6 +3,7 @@
 Локально: TEST_DATABASE_URL=postgresql://… uv run pytest tests/test_db_integration.py
 """
 
+import json
 import os
 import time
 import uuid
@@ -139,3 +140,22 @@ def test_training_and_wave_queries(world: dict[str, Any]) -> None:
         assert active is not None and active[0] == 1
     picks = build_batch(index, profile, Session(), [], n=5, rng=np.random.default_rng(0))
     assert len(picks) == 5
+
+
+def test_onboarding_seeds_taste(world: dict[str, Any]) -> None:
+    user = uid()
+    with psycopg.connect(DSN, autocommit=True) as conn:
+        conn.execute(
+            "INSERT INTO users (id, username, password_hash) VALUES (%s, %s, 'x')", (user, f"new-{user[:6]}")
+        )
+        artist = conn.execute(
+            "SELECT artist_id::text FROM track_artists WHERE track_id = %s", (world["tracks"][0],)
+        ).fetchone()
+        assert artist is not None
+        conn.execute(
+            "INSERT INTO user_profiles_rec (user_id, onboarding) VALUES (%s, %s)",
+            (user, json.dumps({"artist_ids": [artist[0]], "genres": []})),
+        )
+        profile = load_profile(conn, user)
+    assert len(profile.scores) >= 10, "chosen artist's tracks become the initial taste"
+    assert not profile.played