fix(pairing): default bunker_relay to the spire's public event relay, not localhost

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.

pairing.py

@@ -179,26 +179,20 @@ async def pair_spire(
     connection lifecycle out of the orchestration so this is unit-testable
     with a fake client.
 
-    `bunker_relay` / `keystore_passphrase` default to the lnbits bunker
-    settings; injectable for tests. `relays` are the relays the spire will
-    use for its *own* events (kind-21000/30078) — typically the operator's
-    nostrrelay; supplied by the API layer.
+    `relays` are the relays the spire uses for its *own* events
+    (kind-21000/30078) — typically the operator's public nostrrelay; supplied by
+    the API layer. `bunker_relay` (the relay baked into `bunker_url`, where the
+    spire reaches the bunker) defaults to `relays[0]`; `keystore_passphrase`
+    defaults to the lnbits bunker setting. Both injectable for tests.
 
     Raises PairingError on any bunker failure; no state is persisted here
     (the API layer persists on success).
     """
-    relay = (
-        bunker_relay if bunker_relay is not None else settings.lnbits_nsec_bunker_url
-    )
     passphrase = (
         keystore_passphrase
         if keystore_passphrase is not None
         else settings.lnbits_nsec_bunker_keystore_passphrase
     )
-    if not relay:
-        raise PairingError(
-            "LNBITS_NSEC_BUNKER_URL is not set — cannot build a spire bunker connection"
-        )
     if not passphrase:
         raise PairingError(
             "LNBITS_NSEC_BUNKER_KEYSTORE_PASSPHRASE is not set — "
@@ -206,6 +200,14 @@ async def pair_spire(
         )
     if not relays:
         raise PairingError("at least one relay is required for the seed URL")
+    # The relay baked into `bunker_url` is where the *spire* (the remote ATM)
+    # reaches the bunker, so it must be a machine-reachable public URL — NOT
+    # `settings.lnbits_nsec_bunker_url`, which is how the co-located lnbits
+    # reaches the bunker (typically ws://127.0.0.1, unreachable from the ATM —
+    # the localhost-relay /pair gotcha bitspire flagged). Default to the spire's
+    # own event relay (the bunker lives on the same operator relay the spire
+    # publishes to); an explicit `bunker_relay` overrides for split-relay deploys.
+    relay = bunker_relay if bunker_relay else relays[0]
 
     key_name = spire_key_name(machine.id)
     client_name = f"spire-client-{machine.id}"
@@ -250,9 +252,7 @@ async def pair_spire(
     )
 
 
-async def revoke_spire(
-    machine: Machine, *, admin_client: NsecBunkerAdminClient
-) -> int:
+async def revoke_spire(machine: Machine, *, admin_client: NsecBunkerAdminClient) -> int:
     """Revoke a spire's bunker access (the "Revoke spire access" UX,
     aiolabs/spirekeeper#9/#12).
 
@@ -274,6 +274,4 @@ async def revoke_spire(
     except NsecBunkerNotConfiguredError as exc:
         raise PairingError(f"nsecbunkerd is not configured: {exc}") from exc
     except NsecBunkerError as exc:
-        raise PairingError(
-            f"bunker admin RPC failed during revoke: {exc}"
-        ) from exc
+        raise PairingError(f"bunker admin RPC failed during revoke: {exc}") from exc