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>

packages/i18n/locales/en/common.json

@@ -278,5 +278,17 @@
     "cache": "Recent tracks cache",
     "cache_clear": "Clear ({{size}} MB)",
     "cache_pending": "Will be cleared on next launch"
+  },
+  "onboarding": {
+    "title": "What do you like to listen to?",
+    "subtitle": "Pick a few artists and genres to start My Wave. After that it learns from what you play.",
+    "artists": "Artists",
+    "genres": "Genres",
+    "skip": "Skip",
+    "done": "Done — to the wave",
+    "pick_more_one": "Pick {{count}} more artist",
+    "pick_more_few": "Pick {{count}} more artists",
+    "pick_more_many": "Pick {{count}} more artists",
+    "pick_more_other": "Pick {{count}} more artists"
   }
 }