Add scripts/pb-init.sh — an idempotent alpine sh script that authenticates
with the PocketBase admin API and POSTs all required collection schemas
(books, chapters_idx, ranking, progress, audio_cache, app_users) before
any application service starts. 400/422 responses are treated as success so
it is safe to run on every docker compose up.
Wire pb-init into docker-compose.yml:
- pb-init service: alpine:3.19, depends on pocketbase healthy, mounts script
- scraper and ui both depend on pb-init via service_completed_successfully,
guaranteeing collections exist before the first request hits app_users
