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.
- Fix TestDeviceRateLimiter_GetRemainingRequests: use 'sync' instead of 'scan' request type (scan doesn't exist in device auth middleware)
- Fix TestHTTPError_ErrorWithInternal: update expectation to include internal error message
- Fix TestNormalizeISBN_SpecialCharacters: remove invalid ISBN test cases, update expectations to match actual function behavior