Implement book copies system

[NikosPoly]

Currently Book has a simple available boolean which doesn't reflect real world
library scenarios where multiple copies of the same book can exist.

Tasks

• Add totalCopies and availableCopies integer fields to Book
  • totalCopies — total copies the library owns, only changes when library acquires or removes copies
  • availableCopies — how many copies are currently not on loan
• Keep available boolean but sync it automatically from availableCopies > 0 in the service layer, never set manually
• Update LoanService.createLoan() to decrement availableCopies on loan creation
• Update LoanService return logic to increment availableCopies on book return
• Block loan creation when availableCopies = 0

Note

Issue 3 and Issue 5 depend on this being implemented first.