feat(pairing): default seed relay to the nostrclient endpoint + validate relays

Two robustness fixes for on-machine pairing (bitspire-#70), after a QR scan
silently corrupted a seed's relay (ws://→As://) and crash-looped a machine on
an unreachable relay:

- Default relays: when the operator omits `relays`, derive the seed's relay from
  THIS lnbits' own nostrclient proxy endpoint — `<ws(s)>://<host>/nostrclient/api/v1/relay`,
  built from `lnbits_baseurl` (default_relay_endpoint). Operators configure
  upstream relays once in the nostrclient extension (public_ws must be on) and
  every seed points at one stable, operator-independent URL. `relays` is now
  optional on PairMachineData + the /pair endpoint.
- Validate every relay (+ bunker_relay) is a `ws://`/`wss://` URL AND reject
  loopback hosts (localhost/127.0.0.1/::1/0.0.0.0) — a seed is redeemed by a
  REMOTE machine, so a localhost relay is exactly the unreachable case. Catches
  both the ws://→As:// corruption and the localhost /pair gotcha at mint time.

Consumer side (bitspire): parseSpireSeed rejects non-ws relays, and the wizard
gained a "test relay" reachability button before committing.

223 tests pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

views_api.py

@@ -315,8 +315,8 @@ async def api_pair_machine(
     `duration_hours` (optional) time-bounds the token; revoke via the
     sibling `POST .../revoke` endpoint."""
     machine = await _machine_owned_by(machine_id, user.id)
-    if not data.relays:
-        raise HTTPException(HTTPStatus.BAD_REQUEST, "at least one relay is required")
+    # relays may be omitted — pair_spire defaults to this lnbits' nostrclient
+    # proxy endpoint and validates reachability (raises PairingError → 502).
 
     try:
         async with NsecBunkerAdminClient.from_settings() as client: