docs(payments): clarify verification leases and actual invoice expiry #54

Make the server payment guides and API reference explain how paymentTtlMs bounds invoice creation and settlement verification.

Requested in PR #110 review, item 4.

Scope

• Document that paymentTtlMs is milliseconds, must cover the processor's actual invoice lifetime, and needs headroom for pricing and invoice issuance.
• Explain the distinction between a processor's returned offer TTL, the wallet's actual invoice expiry, and authorizationTtlMs after verification.
• State that payments accepted after verification stops require reconciliation; do not imply timeout proves an invoice unpaid.
• Check the NWC, LNbits, and zap examples against the final merged SDK options and apply this guidance in the planned CEP-8 documentation PR.

Acceptance

Guide/reference examples match the merged SDK, the single-process deployment constraint is explicit, and the docs build passes. The local server guide already contains the initial deployment/TTL guidance; final reference and rail-example qualification remain to be published.