v0.1.6

Follow-up to the v0.1.5 account client that makes ClinkWallet pay and track
reliably in both directions:

• Paying out fixed. PayInvoice now sends the node-required amount field:
  0 for a fixed-amount invoice (the invoice amount is used) or the sats to pay
  for an amountless invoice. Previously sending the raw amount was rejected with
  "invoice has value, do not provide amount the the request" and omitting it
  failed with "invalid request body" — both surfaced as an HTTP 520.
• Incoming status fixed. Invoice status now polls the node's
  GetUserOperations API (latestIncomingInvoiceOperations, matched by BOLT11
  and a paidAtUnix timestamp) instead of GetPaymentState, which only tracks
  outgoing payments. Paid invoices now report success instead of hanging
  pending.
• Stale-response guard. Responses are matched by decrypting the event and
  checking its requestId (plus the node pubkey), so a busy node can't satisfy
  a request with a reply meant for another in-flight call.
• Account-first invoice creation. create_invoice prefers the account path
  (mint via NewInvoice) over the noffer offer path when an account is
  configured, so new invoices credit the node balance and are trackable.
• Empty preimages accepted. The node returns an empty preimage for
  internal (same-node) payments; a settled response is now treated as a success
  regardless.
• config.json renamed to CLINK — Recurring Payments (matches the live
  deployment); tests/test_config.py updated to match.

Verified end-to-end on the live instance: paid incoming checkout invoices
report success, and outgoing payments to external wallets settle (service fee
10 sats floor, network fee 0). Full suite: 112 passed.

Full Changelog: v0.1.5...v0.1.6