feat(bookfile): add Contains and OpenReaderAt utilities

backend/internal/bookfile/reader.go

@@ -0,0 +1,37 @@
+package bookfile
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// OpenReaderAt opens a book file and returns a ReaderAt + size.
+// Consolidates the 3 places that open a book file + stat + get ReaderAt.
+func OpenReaderAt(root, rel string) (*os.File, int64, error) {
+	abs := filepath.Join(root, filepath.FromSlash(rel))
+	f, err := os.Open(abs)
+	if err != nil {
+		return nil, 0, err
+	}
+	st, err := f.Stat()
+	if err != nil {
+		f.Close()
+		return nil, 0, err
+	}
+	return f, st.Size(), nil
+}
+
+// Contains reports whether child is inside parent using EvalSymlinks semantics.
+// Falls back to Clean if EvalSymlinks fails (e.g., path doesn't exist yet).
+func Contains(parent, child string) bool {
+	p, err := filepath.EvalSymlinks(parent)
+	if err != nil {
+		p = filepath.Clean(parent)
+	}
+	c, err := filepath.EvalSymlinks(child)
+	if err != nil {
+		c = filepath.Clean(child)
+	}
+	return c == p || strings.HasPrefix(c, p+string(os.PathSeparator))
+}