feat(stellar): Implement invoice system

[zeemscript]

Description

Create a comprehensive invoice system for tracking payment requests. This includes invoice creation, status tracking (draft, sent, paid, overdue), and payment association for better accounting and record-keeping.

Acceptance Criteria

• Create invoice model with line items and totals
• Track invoice status lifecycle (draft, sent, paid, overdue, cancelled)
• Link invoices to Stellar payments upon completion
• Provide CRUD endpoints for invoice management

Files to modify/create

• src/modules/stellar/invoice.service.ts
• src/modules/stellar/invoice.controller.ts
• src/modules/stellar/entities/invoice.entity.ts
• src/modules/stellar/dto/invoice.dto.ts