feat: also publish allow_fiat + fiat_currency in NIP-52 tags

The buyer-side webapp Purchase button needs allow_fiat to know
whether to surface the fiat method, and fiat_currency for the
conversion-preview label. Without these in the published Nostr
event, the buyer would either have to REST-fetch the LNbits event
again (defeats the inventory-sync goal) or guess.

Same backwards-compat reasoning as the four counter tags — tags
are AIO additions outside the NIP-52 spec; unknown tags are
ignored by spec-compliant clients.

- tickets_allow_fiat: "true" when the organizer enabled the fiat
  toggle. Omitted otherwise so the on-the-wire payload stays
  small for the common Lightning-only case.
- tickets_fiat_currency: only emitted when allow_fiat is on
  (otherwise it'd be ambiguous what the value represents).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nostr_publisher.py

@@ -44,11 +44,13 @@ def build_nip52_event(event: Event, pubkey: str) -> NostrEvent:
       start              - unix timestamp (31923) or YYYY-MM-DD (31922)
       end                - same encoding (optional)
       image, location, t (categories) - optional
-      tickets_available  - current remaining capacity (omitted when unlimited)
-      tickets_sold       - running paid-count (always emitted; clients can
-                           derive original_capacity = available + sold)
-      tickets_price      - price_per_ticket (always emitted; 0 means free)
-      tickets_currency   - the currency string
+      tickets_available     - current remaining capacity (omitted when unlimited)
+      tickets_sold          - running paid-count (always emitted; clients can
+                              derive original_capacity = available + sold)
+      tickets_price         - price_per_ticket (always emitted; 0 means free)
+      tickets_currency      - the currency string
+      tickets_allow_fiat    - "true" when fiat checkout is enabled (omitted otherwise)
+      tickets_fiat_currency - the fiat settle currency (only when allow_fiat)
     Content:  event.info
 
     The four ticket_* tags are AIO custom additions outside the NIP-52
@@ -100,6 +102,12 @@ def build_nip52_event(event: Event, pubkey: str) -> NostrEvent:
     tags.append(["tickets_sold", str(event.sold)])
     tags.append(["tickets_price", str(event.price_per_ticket)])
     tags.append(["tickets_currency", event.currency])
+    # Fiat-checkout config — only emitted when allow_fiat is on so
+    # clients can branch the buy UI without re-reading the schema.
+    if event.allow_fiat:
+        tags.append(["tickets_allow_fiat", "true"])
+        if event.fiat_currency:
+            tags.append(["tickets_fiat_currency", event.fiat_currency])
 
     nostr_event = NostrEvent(
         pubkey=pubkey,