Add Support for Custom Invoice Number Prefixes

🔍 Problem Statement

Invoice numbers in Pelcro follow a system-generated format with no option for custom prefixes. Clients who need branded or department-specific invoice numbering (e.g. 'INV-2025-001') must manually rename invoices or track separately.

💡 User Story

As a Pelcro admin, I want to configure a custom prefix for invoice numbers, so that generated invoices follow our organisation's numbering convention without manual renaming.

🎯 Definition of Done (DoD)

A feature is done when:

✔️ Given a configured invoice number prefix (e.g. 'INV-'), when a new invoice is generated, then the invoice number includes the prefix followed by the auto-incremented sequence number.