chore: replace host-path bind mount with named volume for static books

Scraped content is now stored in the 'static_books' Docker named volume
instead of a host bind mount, removing the dependency on STATIC_ROOT and
the need to pre-create ./static/books on the host.

docker-compose.yml

@@ -71,13 +71,12 @@ services:
     ports:
       - "8080:8080"
     volumes:
-      # Mount the host static directory so scraped content is available outside
-      # the container. Create ./static/books on the host first if needed.
-      - "${STATIC_ROOT:-./static/books}:/app/static/books"
+      - static_books:/app/static/books
     healthcheck:
       test: ["CMD", "wget", "-qO-", "http://localhost:8080/health"]
       interval: 15s
       timeout: 5s
       retries: 3
 
-volumes: {}
+volumes:
+  static_books: