feat: monthly invoice generation worker

[frank0277]

Summary

Implements the foundation for monthly invoice generation by adding a dedicated invoice service, a scheduled monthly worker, invoice database migrations, and invoice creation event support.

Changes

• Added InvoiceService for monthly invoice generation logic.

• Added monthlyInvoiceJob worker to generate invoices for closed billing periods.

• Added database migrations for:

  • invoices
  • invoice_line_items

• Extended the event system with invoice_created.

• Added InvoiceCreatedData webhook payload type.

Notes

The implementation is designed to:

• Generate invoices for completed billing periods.
• Support idempotent invoice generation using the billing period identifier.
• Keep invoice creation logic centralized within a dedicated service.
• Provide an event hook for downstream processing and webhook delivery.

Testing

• Verified project changes compile locally as far as possible within the current development environment.
• Manual review of invoice generation flow and event integration.

Checklist

• Added monthly invoice worker
• Added invoice service
• Added database migration
• Added invoice creation event support
• Kept changes isolated to the invoice generation feature
  Closes Add a monthly invoice generation job that closes the previous period #462

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[GBOYEE]

@GBOYEE has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Implement following existing patterns and conventions. Add tests, ensure CI passes, and document any new API surface.

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