Loan creation by ISBN and customerId

[NikosPoly]

Currently creating a loan requires MongoDB ObjectIds for both book and customer
which is not practical in a real library scenario.

Instead of:
{
"bookId": "6a2aa818dbf3dce5994ce296",
"customerId": "6a2ae775e7356a48f90074b6",
...
}

Should accept:
{
"isbn": "9781234567890",
"customerId": "CUST001",
...
}

Tasks

• Update LoanService.createLoan() to accept isbn and customerId as human readable identifiers
• Service resolves isbn to bookId via BookRepository.findByIsbn()
• Service resolves customerId to MongoDB ID via CustomerRepository.findByCustomerId()
• Store MongoDB IDs internally in the Loan document as before

Note

Depends on Issue 11 (findByIsbn) being implemented first.