The seed minted via the Pair UI baked an unreachable bunker relay into
bunker_url. The UI form has no bunker_relay field, so pair_spire fell back to
its default `settings.lnbits_nsec_bunker_url` — which on a deployed instance is
the INTERNAL relay lnbits uses to reach the co-located bunker (e.g.
ws://127.0.0.1:5000/nostrrelay/demo). The remote ATM can't reach localhost, so
connectNewSeed hangs -> BunkerTimeoutError "Signer Unreachable". (Flagged by
bitspire on the demo; the localhost-relay /pair gotcha the coord thread called out.)
Default bunker_relay to the spire's own public event relay (relays[0]) instead:
the bunker lives on the same operator nostrrelay the spire publishes its events
to, so that URL is machine-reachable. An explicit `bunker_relay` still overrides
for split-relay deploys. An empty override now falls back to the same default
rather than raising.
Regression test: with no (or empty) bunker_relay, bunker_url embeds relays[0]
and contains no 127.0.0.1.
NOTE: relays[0] is a pragmatic default; whether the seed should carry multiple
relays / be sourced from the operator's nostrclient relay is a follow-up.
