refactor: drop the SatsPay/watchonly on-chain surface

v1.6.8 sells tickets on-chain through SatsPay charges — a per-purchase
watchonly address, a hosted charge page, and a webhook back into the
extension. We are not adopting that: on-chain goes through the fork's
native LndRestWallet support once core can mint purpose-bound receiving
addresses (aiolabs/lnbits#53). This is the parity note on #41, applied
as its own commit so the merge it follows stays a faithful diff of what
upstream shipped.

Removed:
- services: the five SatsPay/watchonly HTTP clients (and with them the
  only uses of httpx and typing.Any in that module)
- views_api: _get_watchonly_status, GET /events/onchain/status, the
  SatsPay charge branch of api_ticket_create, POST
  /tickets/{id}/satspay-webhook, PUT /tickets/{hash}/onchain-confirm
- models: EventExtra.onchain_{enabled,wallet_id,zeroconf,fasttrack},
  TicketExtra.satspay_charge_id, TicketPaymentRequest.satspay_charge_url
- frontend: the organiser's "Onchain payments" panel and its watchonly
  wallet picker, the per-ticket confirm-onchain button, the SatsPay
  charge-page redirect, and the wallet-status fetch behind them

`onchain` is no longer an accepted payment_method — there is no rail to
fulfil it until #41 lands, so accepting it could only fail later.

Kept as vocabulary for #41, in upstream's field names so it needs no
migration: TicketExtra.onchain / onchain_address and
TicketPaymentRequest.onchain_amount_sat.

35 routes register with no duplicates; ruff, black, prettier clean; 97
tests pass; mypy error set still identical to HEAD's baseline.

static/js/index.vue

@@ -408,18 +408,6 @@
                   >
                     <q-tooltip>Resend ticket email</q-tooltip>
                   </q-btn>
-                  <q-btn
-                    v-if="props.row.extra?.onchain && !props.row.paid"
-                    flat
-                    dense
-                    size="xs"
-                    @click="confirmOnchainTicket(props.row)"
-                    icon="currency_bitcoin"
-                    color="orange"
-                    class="q-ml-xs"
-                  >
-                    <q-tooltip>Confirm onchain payment</q-tooltip>
-                  </q-btn>
                 </q-td>
 
                 <q-td v-for="col in props.cols" :key="col.name" :props="props">
@@ -909,55 +897,6 @@
             </div>
           </q-expansion-item>
 
-          <q-expansion-item
-            group="advanced"
-            icon="view_in_ar"
-            label="Onchain payments"
-          >
-            <div class="q-mt-lg">
-              <div class="text-caption q-mb-md">
-                Accept Bitcoin onchain payments. Requires the
-                <strong>SatsPay</strong> and <strong>Watchonly</strong>
-                extensions to be installed and enabled.
-              </div>
-              <q-toggle
-                v-model="formDialog.data.extra.onchain_enabled"
-                label="Accept onchain payments"
-                left-label
-                @update:model-value="val => val && loadOnchainWallets()"
-              ></q-toggle>
-              <q-select
-                v-if="formDialog.data.extra.onchain_enabled"
-                filled
-                dense
-                class="q-mt-md"
-                v-model="formDialog.data.extra.onchain_wallet_id"
-                label="Watchonly wallet"
-                :options="
-                  onchainWallets.map(w => ({
-                    label: w.title || w.id,
-                    value: w.id
-                  }))
-                "
-                emit-value
-                map-options
-                hint="Bitcoin watchonly wallet for receiving onchain payments"
-              ></q-select>
-              <div v-if="formDialog.data.extra.onchain_enabled" class="q-mt-md">
-                <q-toggle
-                  v-model="formDialog.data.extra.onchain_zeroconf"
-                  label="Zero-conf (accept unconfirmed transactions)"
-                  left-label
-                ></q-toggle>
-                <q-toggle
-                  v-model="formDialog.data.extra.onchain_fasttrack"
-                  label="Fasttrack (treat pending as paid)"
-                  left-label
-                ></q-toggle>
-              </div>
-            </div>
-          </q-expansion-item>
-
           <div class="row q-mt-lg">
             <q-btn
               v-if="formDialog.data.id"