Feature Request: End Client ("Sub-Client") relationship for subcontracted work across Clients, Projects, Tasks, and Invoices

What version of Invoice Ninja are you running? v5 What environment are you running? Docker via Cloudron Have you searched existing issues/requests? Yes. Screenshots If applicable, add screenshots to help explain your request/question. Additional context Add any other context about the request/question here. Summary Add an optional End Client (also called a sub-client, ultimate client, or project owner) relationship that can be attached to a Client, Project, Task, and Invoice. Today an invoice answers one question: *who do I bill?* For anyone working as a subcontractor, there is a second question that matters just as much: *whose work is this actually for?* Invoice Ninja has no first-class way to record that second party, so the information ends up in project names, free-text custom fields, or nowhere at all. The ask is a nullable, self-referential link from an entity to another Client record, surfaced on the invoice PDF, filterable in list views, and rolled up in reports. ---…