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>

internal/library/library.go

@@ -19,8 +19,11 @@ type Service struct {
 	booksDir string
 }
 
-func New(booksDir string) *Service {
-	return &Service{booksDir: booksDir}
+func New(booksDir string) (*Service, error) {
+	if err := os.MkdirAll(booksDir, 0o755); err != nil {
+		return nil, fmt.Errorf("books dir: %w", err)
+	}
+	return &Service{booksDir: booksDir}, nil
 }
 
 func (s *Service) BooksDir() string { return s.booksDir }