Enhance NormalizeISBN to validate and convert ISBNs:
- Validate length (10 or 13 digits), return error if invalid
- Convert ISBN-10 to ISBN-13 by prefixing '978' and recalculating checksum
- Add NormalizeISBNSafe for backward compatibility in scanners
This ensures all ISBNs stored in database are valid ISBN-13 format.
- 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
john-okeefe