fix(payments): poll NWC wallet for pending invoices (closes #89)

[v0l]

Problem

Normal top-up invoices (payment_type = 0) created against an NWC backend are paid in the user's wallet but never marked paid or credited — user balance stays flat and Core spams Balance has run out. See #89.

Root cause

NWCNode::subscribe_invoices detected settlements only through NIP-47 push notifications (subscribe_to_notifications / PaymentReceived). NIP-47 notifications is an optional wallet capability — many wallets/relays never deliver it (or drop the subscription silently). Unlike the LNURLNode backend, the NWC path had no polling fallback, so paid invoices were stranded with is_paid = 0.

The existing reconcile_expired_nwc_payments only helps invoices already past expiry, and only as a one-off --reconcile-payments CLI run — it doesn't fix live traffic.

Fix

Add a 10s polling fallback in NWCNode::subscribe_invoices alongside the notification path:

• pulls db.get_pending_payments()
• looks up each via conn.lookup_invoice { payment_hash }
• emits InvoiceUpdate::Settled (with preimage) when settled_at.is_some()

Both paths funnel through the existing try_complete_payment → db.complete_payment, which is idempotent (where is_paid = false), so overlap between the notification and polling paths is harmless. This mirrors the existing LNURLNode poller and the reconcile_expired_nwc_payments settlement check.

NWCNode now carries a ZapStreamDb handle (like LNURLNode); NWCNode::new and the create_lightning NWC arm are updated accordingly.

Testing

• cargo check -p zap-stream — clean, no new warnings.

Closes #89