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.

tests/test_pairing.py

@@ -218,17 +218,29 @@ def test_malformed_token_raises():
         _pair(bunker)
 
 
-def test_missing_relay_or_passphrase_raises():
-    with pytest.raises(PairingError, match="LNBITS_NSEC_BUNKER_URL"):
-        asyncio.run(
+def test_bunker_relay_defaults_to_spire_event_relay():
+    """No explicit bunker_relay -> the relay baked into bunker_url is the spire's
+    own public event relay (relays[0]), NOT lnbits's internal bunker URL. This
+    is the localhost-relay /pair gotcha: a UI-minted seed (the form has no
+    bunker_relay field) must embed a machine-reachable relay, not ws://127.0.0.1.
+    An empty bunker_relay falls back to the same default."""
+    from urllib.parse import quote
+
+    for empty in (None, ""):
+        result = asyncio.run(
             pair_spire(
                 _machine(),
                 relays=_RELAYS,
-                admin_client=FakeBunker(),
-                bunker_relay="",
+                admin_client=FakeBunker(token_secret="s"),  # pragma: allowlist secret
+                bunker_relay=empty,
                 keystore_passphrase=_PASSPHRASE,
             )
         )
+        assert f"relay={quote(_RELAYS[0], safe='')}" in result.bunker_url
+        assert "127.0.0.1" not in result.bunker_url
+
+
+def test_missing_relay_or_passphrase_raises():
     with pytest.raises(PairingError, match="PASSPHRASE"):
         asyncio.run(
             pair_spire(