Peering: Lightning invoices for paid peering — issue, settle, renew, lapse

Part of the relay peering work in #931.

Scope

The source (selling) side of paid peering, using nostream's existing payments processors — the same machinery as admission fees:

When a configured peer with paid terms authenticates, the source relay issues a BOLT11 invoice for the amount/period from its advertised terms.
Settlement is detected via the existing processor callbacks; on settlement the peer's feed access activates until the end of the paid period.
Before expiry, the source offers a renewal invoice.
If the period lapses unpaid, the feed is suspended — stored data and sync state are kept, so resuming is a payment, not a re-sync from zero.

A paid peering relationship completes end-to-end: authenticate → invoice → settlement → feed active for exactly the paid period.
An unpaid lapse suspends the feed; paying the renewal resumes it without re-reconciliation.
Invoice/settlement state per peer is recorded for the admin console.