feat(invoices): implement self-contained InvoicesModule for invoice generation and tracking

[lishmanTech]

• add Invoice entity with invoiceNumber, orderId, paymentId, items JSONB, totals
• implement invoice number generator INV-YYYYMM-XXXX
• add DTOs for invoice creation and filtering
• implement pagination and filtering by status/date
• add endpoints for download payload, send, and void invoice
• register invoices module with TypeORM repository

closes #644

[vercel]

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