Add integration test for the settlement endpoint distributing returns to all investors and updating invoice status to settled

[Chucks1093]

Summary

The settlement endpoint should calculate each investor's pro-rata return, transfer the amounts via Soroban, and transition the invoice status to settled. An integration test should fund an invoice with multiple investors and verify all returns are distributed and the final status is correct.

Scope

• Seed a funded invoice (face value 9000) with three equal investors (3000 each)
• Call the settlement endpoint as the seller
• Assert each investor's return record shows 3000 (equal split, no remainder)
• Assert the invoice status is settled after the call
• Assert calling settlement a second time returns 409 with invoice_already_settled

Acceptance Criteria

• Each investor receives correct pro-rata return
• Invoice status transitions to settled
• Second settlement attempt returns 409 with invoice_already_settled
• Seller cannot settle an invoice they do not own (403)

ETA: 12 hours

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