fix(lnd): pay through Router.SendPaymentV2 — LND 0.21 removed the old route

LND 0.21.2 removed the deprecated Lightning.SendPaymentSync REST route
(/v1/channels/transactions). The backend still called it, so every
Lightning send answered literal HTTP 404 and the wallet UI reported
'Payment failed: Not Found' fleet-wide right after the pin bump —
receive worked, which made it look intermittent.

Pay through the supported Router.SendPaymentV2 route (/v2/router/send)
instead, keeping the existing contract with the UI:
- single-record responses (no_inflight_updates) unwrapped from the
  grpc-gateway result envelope, transport errors from the nested error
- a slow multi-hop payment still resolves as pending + payment hash
  (only LND may declare failure), never a false 'Payment failed'
- LND's failure_reason codes translated to the same plain-language
  advice, invoice-expiry still says 'ask for a fresh invoice'

Guard it at the gate: tests/lifecycle/bats/lnd-api-compat.bats POSTs a
deliberately-invalid invoice to /v2/router/send on the RUNNING LND and
fails if the route answers 404 — the image/backend skew that shipped
silently last time because no test ever spoke the payment endpoint.
Also bumps the stale lnd image expectation in remote-lifecycle.sh.

tests/lifecycle/remote-lifecycle.sh

@@ -136,7 +136,7 @@ image_for() {
     bitcoin-knots) echo "source.archipelago-foundation.org/lfg2025/bitcoin-knots:latest" ;;
     bitcoin-core) echo "docker.io/bitcoin/bitcoin:28.4" ;;
     btcpay-server) echo "docker.io/btcpayserver/btcpayserver:2.4.2" ;;
-    lnd) echo "source.archipelago-foundation.org/lfg2025/lnd:v0.18.4-beta" ;;
+    lnd) echo "source.archipelago-foundation.org/lfg2025/lnd:v0.21.2-beta" ;;
     mempool) echo "source.archipelago-foundation.org/lfg2025/mempool-frontend:v3.0.0" ;;
     homeassistant) echo "source.archipelago-foundation.org/lfg2025/home-assistant:2024.1" ;;
     grafana) echo "source.archipelago-foundation.org/lfg2025/grafana:10.2.0" ;;