Add ISBN based book operations

[NikosPoly]

Currently all book operations require the MongoDB ObjectId which is not practical
in a real library scenario. Librarians work with ISBNs, not internal database IDs.

Tasks

• Add findByIsbn(String isbn) to BookRepository
• Add the following ISBN based endpoints to BookController and BookService:
  • GET /books/isbn/{isbn} — find book by ISBN
  • PUT /books/isbn/{isbn} — full update by ISBN
  • PATCH /books/isbn/{isbn} — partial update by ISBN
  • DELETE /books/isbn/{isbn} — delete by ISBN

Note

MongoDB ID based endpoints remain available for internal/system use.
ISBN based endpoints are for real world library usage.