cleanup: remove outdated documentation and summaries

- Remove API_TESTING_SUMMARY.md, IMPLEMENTATION_SUMMARY.md
- Remove MIGRATION_PROGRESS.md, SECURITY_*.md files
- Clean up temporary documentation files from previous sessions
- Repository now focused on current working code

internal/utils/isbn.go

@@ -0,0 +1,16 @@
+package utils
+
+import (
+	"regexp"
+)
+
+// NormalizeISBN removes hyphens and spaces from ISBN to standardize format
+// Handles ISBN-10 and ISBN-13 formats
+func NormalizeISBN(isbn string) string {
+	if isbn == "" {
+		return ""
+	}
+
+	// Remove hyphens and spaces, return only digits and X (for ISBN-10)
+	return regexp.MustCompile(`[-\s]`).ReplaceAllString(isbn, "")
+}