Peering: advertise peering terms and scopes in the NIP-11 document #932

Part of the relay peering work in #931.

Scope

Add a peering section to the relay information document nostream serves:

• Whether peering is offered at all.
• Offered scopes: kinds and time ranges available as feeds.
• Terms reusing the existing NIP-11 fee-object shape (amount, unit, period); settlement-free peering must be expressible (no fee entry).
• Peer contact/endpoint for establishing a relationship.
• Minimum proof-of-work (NIP-13 bits) required on peering requests — receiver-set policy, advertised like min_pow_difficulty in the existing limitations. Absent or 0 means none. Lets a relay triage unsolicited peering requests (see Peering: owner-attested peering requests — the operator co-signs, staking their reputation #940) without a global constant.

Configuration-driven from the relay settings, validated like the rest of the settings document. This is the discovery surface other relays read before connecting — it expresses terms, it does not accept peers.

Acceptance

• A nostream relay can publish a peering section covering settlement-free and paid configurations, including a minimum request PoW.
• Invalid peering configuration is rejected by settings validation with a clear error.
• Docs: peering fields documented alongside the other NIP-11 settings.