mvp: enforce invoice expiry on verify and periodic sweep

[CemAyyildiz]

Goal

Make invoice expiry real on the MVP demo backend: past-due PENDING invoices must transition to EXPIRED, and POST /api/invoices/:id/verify must reject settlement with INVOICE_EXPIRED instead of accepting Horizon-matched payments on stale invoices.

Context

The frontend already surfaces expiry (frontend/app/pay/[id]/page.tsx countdown + EXPIRED UI; dashboard expired filter). Backend storage implements the transition but the MVP server never calls it:

• backend/src/storage/memory-storage.ts — markExpiredInvoices() transitions PENDING → EXPIRED when expiresAt < now
• backend/src/services/invoice-memory.service.ts — exposes markExpiredInvoices()
• backend/src/utils/verify-errors.ts — defines INVOICE_EXPIRED + message
• backend/src/server-mvp.ts — verify only checks status !== 'PENDING'; never reads expiresAt and never calls markExpiredInvoices()
• backend/tests/memory-storage.test.ts — unit coverage for expiry in isolation

The Postgres path runs a periodic sweep in payment-monitor.service.ts; the MVP demo path does not.

Acceptance criteria

• server-mvp.ts runs invoiceService.markExpiredInvoices() on a sensible interval after startup (mirror ~60s cadence in payment-monitor.service.ts)
• POST /api/invoices/:id/verify calls expiry check before Horizon lookup; past-due invoices return HTTP 400 with code: INVOICE_EXPIRED
• GET /api/invoices/:id and list/stats reflect EXPIRED after sweep (no stale PENDING past expiresAt)
• Backend tests prove: past-due invoice → EXPIRED after sweep; verify on expired → INVOICE_EXPIRED; valid pending verify still succeeds
• Update docs/VERIFY.md error table for expiry rejection on MVP path

Out of scope

• Pay-page UI changes (already handles EXPIRED)
• Automatic payment detection / Horizon streaming
• Proof PDF / quittance output changes
• Shared TypeScript API contract types
• Postgres payment-monitor.service.ts refactor

Conflict rule

• Do not edit: frontend/app/pay/[id]/page.tsx, frontend/lib/export.ts, frontend/lib/mock-api.ts

Complexity

Complexity: Medium — 150 points (Stellar Wave)

[codewithzubair07]

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[Kingsilva36]

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[Meeynarh1]

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I'd love to work on this issue. I'll keep the implementation simple, clean, and well-tested.

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[egekoca]

@egekoca has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

give me ser

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