feat: add routes-b duplicate invoice endpoint

[floxxih]

Summary

• add POST /api/routes-b/invoices/[id]/duplicate to create a new pending invoice from an existing one
• enforce auth and ownership checks with 401/403/404 behavior for unauthorized, forbidden, and missing records
• generate a fresh invoice number and payment link while omitting dueDate/paidAt/cancelledAt from the copied invoice state

Closes #371

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