feat: add pb-init-v2.sh for v2 stack; wire into docker-compose-new.yml

Minimal PocketBase bootstrap for the v2 stack (backend + runner + ui-v2).
Creates only the 6 collections actually used by v2:
  books, chapters_idx, ranking, progress, scraping_tasks, audio_jobs

Drops v1-only collections (app_users, user_settings, audio_cache,
book_comments, comment_votes, user_library, user_sessions,
user_subscriptions) and unused fields (date_label, user_id/audio_time
on progress).  heartbeat_at is included in create_collection from the
start and also covered by ensure_field for existing instances.

docker-compose-new.yml pb-init service now mounts pb-init-v2.sh.

docker-compose-new.yml

@@ -65,7 +65,7 @@ services:
       POCKETBASE_ADMIN_EMAIL: "${POCKETBASE_ADMIN_EMAIL:-admin@libnovel.local}"
       POCKETBASE_ADMIN_PASSWORD: "${POCKETBASE_ADMIN_PASSWORD:-changeme123}"
     volumes:
-      - ./scripts/pb-init.sh:/pb-init.sh:ro
+      - ./scripts/pb-init-v2.sh:/pb-init.sh:ro
     entrypoint: ["sh", "/pb-init.sh"]
 
   # ─── Backend API ──────────────────────────────────────────────────────────────