feat(infra): add pb-init sidecar to create PocketBase collections on startup

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

docker-compose.yml

@@ -59,6 +59,22 @@ services:
       timeout: 5s
       retries: 5
 
+  # ─── PocketBase collection bootstrap ────────────────────────────────────────
+  # One-shot init container: creates all required collections via the admin API
+  # and exits. Idempotent — safe to run on every `docker compose up`.
+  pb-init:
+    image: alpine:3.19
+    depends_on:
+      pocketbase:
+        condition: service_healthy
+    environment:
+      POCKETBASE_URL: "http://pocketbase:8090"
+      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
+    entrypoint: ["sh", "/pb-init.sh"]
+
   # ─── Browserless ────────────────────────────────────────────────────────────
   browserless:
     image: ghcr.io/browserless/chromium:latest
@@ -94,6 +110,8 @@ services:
     #container_name: libnovel-scraper
     restart: unless-stopped
     depends_on:
+      pb-init:
+        condition: service_completed_successfully
       pocketbase:
         condition: service_healthy
       minio:
@@ -142,6 +160,8 @@ services:
     container_name: libnovel-ui
     restart: unless-stopped
     depends_on:
+      pb-init:
+        condition: service_completed_successfully
       scraper:
         condition: service_healthy
       pocketbase: