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>

api/openapi.yaml

@@ -1256,6 +1256,38 @@ paths:
           description: Снято
         default:
           $ref: "#/components/responses/Problem"
+  /api/v1/onboarding:
+    get:
+      operationId: getOnboarding
+      tags: [wave]
+      summary: Онбординг вкуса — исполнители и жанры библиотеки для выбора
+      description: |
+        `required` — показать онбординг: пользователь его не проходил и не пропускал, а своей истории почти нет.
+      responses:
+        "200":
+          description: Варианты и состояние
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/Onboarding"
+        default:
+          $ref: "#/components/responses/Problem"
+  /api/v1/me/onboarding:
+    put:
+      operationId: saveOnboarding
+      tags: [wave]
+      summary: Сохранить выбор онбординга (или пропуск — пустые списки)
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/OnboardingChoice"
+      responses:
+        "204":
+          description: Сохранено
+        default:
+          $ref: "#/components/responses/Problem"
   /api/v1/wave/options:
     get:
       operationId: getWaveOptions
@@ -1442,6 +1474,59 @@ components:
           schema:
             $ref: "#/components/schemas/Problem"
   schemas:
+    Onboarding:
+      type: object
+      required: [required, artists, genres]
+      properties:
+        required:
+          type: boolean
+        artists:
+          type: array
+          items:
+            type: object
+            required: [id, name, track_count, cover_url, cover_colors]
+            properties:
+              id:
+                type: string
+                format: uuid
+              name:
+                type: string
+              track_count:
+                type: integer
+              cover_url:
+                type: string
+                nullable: true
+                description: Обложка самого большого альбома исполнителя
+              cover_colors:
+                type: array
+                items:
+                  type: string
+        genres:
+          type: array
+          items:
+            type: object
+            required: [name, track_count]
+            properties:
+              name:
+                type: string
+              track_count:
+                type: integer
+    OnboardingChoice:
+      type: object
+      required: [artist_ids, genres]
+      properties:
+        artist_ids:
+          type: array
+          maxItems: 50
+          items:
+            type: string
+            format: uuid
+        genres:
+          type: array
+          maxItems: 30
+          items:
+            type: string
+            maxLength: 100
     UserRef:
       type: object
       required: [id, username]