Auto-create books directory, log upload errors, document Docker deployment

- library.New now creates BOOKS_DIR (MkdirAll) on startup so uploads
  don't fail when the directory is missing on a fresh bind mount
- uploadSubmit logs the underlying OS error on save/write failures
- README: document docker-compose deployment, container UID/GID (100/101)
  for bind-mounted ./data permissions, and running the admin CLI via
  docker compose exec

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

cmd/server/main.go

@@ -21,7 +21,10 @@ func main() {
 	}
 	defer store.Close()
 
-	lib := library.New(booksDir)
+	lib, err := library.New(booksDir)
+	if err != nil {
+		log.Fatalf("books dir: %v", err)
+	}
 	h := web.NewHandler(lib, store)
 
 	mux := http.NewServeMux()