Implement payment expiry worker`

[Nemenwq]

Dependencies: Issue #27, Issue #30, Issue #55, Contract Issue #5. For webhook delivery, see Issue #36.\n\nNote: Webhook sending can be stubbed initially.\n

Context:
Pending payments must expire after a timeout (15 minutes per spec).

Scope:

• In scope: Worker job scheduling + status update + webhook.
• Out of scope: Distributed cron coordination beyond BullMQ.

Tasks:

• Decide scheduling strategy:
  • schedule a delayed job at payment creation time, or
  • run a periodic scan for expired PENDING payments
• Update payment status to EXPIRED only if it is still PENDING (race-safe with confirmation).
• Emit the appropriate webhook event record for delivery (Issue Implement webhook delivery with retry logic` #36).

Implementation Notes:

• Use DB-level guards/transactions to avoid expiring a payment that just confirmed.
• Record expired_at for audit/debugging.

Files: backend/payment_service/src/workers/