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/pipu_rec/wave/store.py

@@ -156,8 +156,11 @@ def load_profile(conn: psycopg.Connection[Any], user_id: str) -> Profile:
         )
     }
     row = conn.execute(
-        "SELECT exploration, taste_cf::text, bandit FROM user_profiles_rec WHERE user_id = %s", (user_id,)
+        "SELECT exploration, taste_cf::text, bandit, onboarding FROM user_profiles_rec WHERE user_id = %s",
+        (user_id,),
     ).fetchone()
+    if row and row[3] and len(scores) < ONBOARDING_UNTIL:
+        apply_onboarding(conn, scores, row[3])
     exploration = float(row[0]) if row else 0.15
     taste_cf = parse_vector(row[1]) if row else None
     bandit = {str(k): [float(x) for x in v] for k, v in (row[2] or {}).items()} if row else {}
@@ -179,6 +182,36 @@ def load_profile(conn: psycopg.Connection[Any], user_id: str) -> Profile:
     )
 
 
+# Пока своих оценок меньше, выбор онбординга задаёт начальный вкус (docs/recommendations.md#7).
+ONBOARDING_UNTIL = 30
+ONBOARDING_LIMIT = 300
+
+
+def apply_onboarding(conn: psycopg.Connection[Any], scores: dict[str, float], choice: dict[str, Any]) -> None:
+    """Псевдооценки: треки выбранных исполнителей +1, выбранных жанров +0.5 (до ONBOARDING_LIMIT треков).
+    Собственные оценки не перезаписываются; в «слышанное» эти треки не попадают."""
+    artists = [str(a) for a in choice.get("artist_ids") or []]
+    genres = [str(g).lower() for g in choice.get("genres") or []]
+    pseudo: dict[str, float] = {}
+    if artists:
+        for (tid,) in conn.execute(
+            "SELECT DISTINCT track_id::text FROM track_artists WHERE artist_id::text = ANY(%s) LIMIT %s",
+            (artists, ONBOARDING_LIMIT),
+        ):
+            pseudo[tid] = pseudo.get(tid, 0.0) + 1.0
+    if genres:
+        for (tid,) in conn.execute(
+            """
+            SELECT DISTINCT tg.track_id::text FROM track_genres tg JOIN genres g ON g.id = tg.genre_id
+            WHERE lower(g.name) = ANY(%s) LIMIT %s
+            """,
+            (genres, ONBOARDING_LIMIT),
+        ):
+            pseudo[tid] = pseudo.get(tid, 0.0) + 0.5
+    for tid, v in pseudo.items():
+        scores.setdefault(tid, v)
+
+
 def seed_tracks(conn: psycopg.Connection[Any], seed_type: str, seed_id: str) -> list[str]:
     """Треки, от которых запущена волна: трек, альбом или исполнитель."""
     queries = {