Block payment requests for rejected invoices

[phucnguyen1707]

Summary

• Restrict invoice payment-request creation to payable invoice statuses only: sent and expired
• Keep paid invoices on the existing already-paid error path
• Add a regression test proving rejected invoices do not create CoinPay payment requests

Why

Rejected invoices are blocked in the UI, but the payment-request endpoint could still create a fresh CoinPay request for status=rejected when called directly. That could revive an invoice the payer already rejected and confuse payment records.

Tests

• pnpm test:run src/app/api/gigs/[id]/invoice/[invoiceId]/payment-request/route.test.ts
• pnpm type-check
• pnpm lint
• pre-commit hook: lint, type-check, full test suite, build