feat(pairing): slim the spire seed + carry lnbits_npub (bitspire-#70)

Mint the new-shape seed the bitspire consumer now expects: the pubkey rides
once as spire_npub (consumer derives the hex + reconstructs bunker_url from
bunker_secret + bunker_relay|relays[0]), and lnbits_npub is embedded so a paired
machine reaches this lnbits' nostr-transport with nothing else provisioned.

- build_seed_url emits {spire_npub, lnbits_npub, bunker_secret, relays} and
  bunker_relay only when it differs from relays[0] (omitted in the common case).
  Drops spire_pubkey + the full bunker_url from the payload.
- pair_spire reads settings.nostr_transport_public_key, hex_to_npub's it, and
  raises PairingError when it's empty (transport not running → can't mint a
  self-sufficient seed). bunker_url is still returned in PairResult for operator
  display / audit; only the seed stops embedding it.

Consumer side: bitspire packages/nostr-client/src/seed.ts + the #70 machine
wiring. Kept as v: 1 (redefined in place; no shipped seed to preserve).

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

pairing.py

@@ -23,15 +23,21 @@ We deliberately do NOT run the connect/eager-bind step here: the spire is the
 NIP-46 client, so the binding must happen spire-side with the spire's own
 client keypair. spirekeeper only mints + packages.
 
-Seed URL wire format (contract shared with bitspire#52):
+Seed URL wire format (contract shared with bitspire#52, slimmed in bitspire#70):
 
     spire-seed:v1:<base64url(json)>      json = {
         "v": 1,
-        "spire_npub":   "npub1…",         # the bunker-minted spire identity
-        "spire_pubkey": "<64-hex>",       # same key, hex (consumer convenience)
-        "bunker_url":   "bunker://<spire_pubkey>?relay=<bunker_relay>&secret=<sec>",
-        "relays":       ["wss://…"],      # relays for the spire's own events
+        "spire_npub":    "npub1…",        # the bunker-minted spire identity
+        "lnbits_npub":   "npub1…",        # this lnbits' nostr-transport server id
+        "bunker_secret": "<sec>",         # one-shot NIP-46 connect token
+        "relays":        ["wss://…"],     # relays for the spire's own events
+        "bunker_relay":  "wss://…",       # OPTIONAL — omitted when == relays[0]
     }
+
+The pubkey is carried ONCE, as an npub: the consumer derives spire_pubkey (hex)
+and reconstructs the bunker:// URL from spire_npub + bunker_relay|relays[0] +
+bunker_secret. `lnbits_npub` lets a paired machine reach this lnbits' transport
+with nothing else provisioned. See bitspire packages/nostr-client/src/seed.ts.
 """
 
 from __future__ import annotations
@@ -48,6 +54,7 @@ from lnbits.core.services.nsec_bunker import (
 )
 from lnbits.core.signers.remote_bunker import ensure_policy
 from lnbits.settings import settings
+from lnbits.utils.nostr import hex_to_npub
 from pydantic import BaseModel
 
 from .models import Machine
@@ -136,15 +143,27 @@ def _recover_token(tokens: list[dict], client_name: str) -> str:
 
 
 def build_seed_url(
-    *, spire_npub: str, spire_pubkey_hex: str, bunker_url: str, relays: list[str]
+    *,
+    spire_npub: str,
+    lnbits_npub: str,
+    bunker_secret: str,
+    relays: list[str],
+    bunker_relay: str | None = None,
 ) -> str:
-    payload = {
+    """Build the slim seed URL (bitspire#70). The pubkey rides once as
+    `spire_npub`; the consumer derives the hex + reconstructs `bunker_url` from
+    `bunker_secret` + `bunker_relay` (or `relays[0]`). `bunker_relay` is emitted
+    only when it differs from `relays[0]`, keeping the common case one field
+    lighter."""
+    payload: dict = {
         "v": 1,
         "spire_npub": spire_npub,
-        "spire_pubkey": spire_pubkey_hex,
-        "bunker_url": bunker_url,
+        "lnbits_npub": lnbits_npub,
+        "bunker_secret": bunker_secret,
         "relays": relays,
     }
+    if bunker_relay and relays and bunker_relay != relays[0]:
+        payload["bunker_relay"] = bunker_relay
     blob = (
         base64.urlsafe_b64encode(json.dumps(payload, separators=(",", ":")).encode())
         .decode()
@@ -233,15 +252,31 @@ async def pair_spire(
     token = _recover_token(tokens, client_name)
     _, _, secret = token.partition("#")
 
+    # The spire needs THIS lnbits' nostr-transport server identity to reach the
+    # backend from the seed alone (bitspire#70). The transport sets
+    # `settings.nostr_transport_public_key` at startup; if it's empty the
+    # transport isn't running, so we can't mint a self-sufficient seed.
+    lnbits_pubkey_hex = settings.nostr_transport_public_key
+    if not lnbits_pubkey_hex:
+        raise PairingError(
+            "LNbits nostr transport has no server pubkey "
+            "(settings.nostr_transport_public_key is empty) — is the transport "
+            "running? Cannot mint a self-sufficient seed."
+        )
+    lnbits_npub = hex_to_npub(lnbits_pubkey_hex)
+
+    # bunker_url is still returned in PairResult (operator display / audit); the
+    # seed itself no longer embeds it — the consumer reconstructs it.
     bunker_url = (
         f"bunker://{spire_pubkey_hex}?relay={quote(relay, safe='')}"
         f"&secret={quote(secret, safe='')}"
     )
     seed_url = build_seed_url(
         spire_npub=spire_npub,
-        spire_pubkey_hex=spire_pubkey_hex,
-        bunker_url=bunker_url,
+        lnbits_npub=lnbits_npub,
+        bunker_secret=secret,
         relays=relays,
+        bunker_relay=relay,
     )
     return PairResult(
         spire_npub=spire_npub,