Add a loan renewal endpoint

## What problem does this solve?

A loan currently has one due date set at borrow time (`loanPeriodDays`), with no way to extend it. In practice borrowers want to renew a loan (extend the due date) instead of returning and immediately re-borrowing the same copy — which today would briefly free the copy for someone else and is a clunky two-call workaround anyway.

## Proposed solution

- Add `POST /api/v1/loans/{id}/renew` (JWT-gated) that extends `dueDate` by another `loanPeriodDays` (or an explicit override), inside a transaction, and rejects (`409`) if the loan is already returned or if a hold exists on the book (see the holds-queue feature) — renewals shouldn't skip a waiting line.
- Track a `renewalCount` on the loan and support an optional `MAX_RENEWALS` config cap (returning `409` once exceeded).
- Add a `loans renew ` CLI subcommand alongside the existing `loans borrow`/`loans return`.

## Alternatives considered

Return + immediately re-borrow via the existing two endpoints — already possible, but it's not atomic (another borrower could grab the copy in between) and doesn't preserve loan history as a single continuous loan.

## Area

Loans (borrow / return)

## Priority

Would significantly improve on the current borrow/return-only model — renewal is one of the most common actions in any real lending system.