feat: per-event payment methods (extra.payment_methods)

Organizers pick which rails an event accepts — Lightning, card (fiat) or
both — instead of a bare "allow fiat" toggle. `extra.payment_methods`
uses the field name upstream v2 (lnbits/events#64) introduces so the
eventual rebase merges cleanly; an empty list keeps the legacy rule
(Lightning always, fiat when allow_fiat), and allow_fiat stays the
fiat-currency carrier, kept in lockstep on save. The effective list is
published as the NIP-52 tag `tickets_payment_methods` so clients render
exactly the buttons the purchase endpoint will accept, and the buyer page
defaults to the first accepted rail (a card-only event never submits
"lightning").

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EYwoAkZZmXMMmaBp4WGUBo

static/js/display.vue

@@ -90,20 +90,18 @@
               :hint="`If minimum tickets (${event.extra?.min_tickets}) are not met, refund will be sent.`"
             ></q-input>
             <div class="row q-col-gutter-md q-pt-lg items-center">
-              <div v-if="allowFiatCheckout" class="col-auto">
+              <div v-if="paymentMethods.length > 1" class="col-auto">
                 <q-option-group
                   v-model="formDialog.data.payment_method"
                   inline
-                  :options="[
-                    {label: 'Lightning', value: 'lightning'},
-                    {
-                      label: fiatCheckoutLabel,
-                      value: 'fiat'
-                    }
-                  ]"
+                  :options="paymentMethodOptions"
                 ></q-option-group>
               </div>
-              <div :class="allowFiatCheckout ? 'col-12 col-md-3' : 'col-12'">
+              <div
+                :class="
+                  paymentMethods.length > 1 ? 'col-12 col-md-3' : 'col-12'
+                "
+              >
                 <q-input
                   filled
                   dense