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.js

@@ -272,8 +272,7 @@ window.PageEvents = {
             promo_codes: []
           }
         }
-      },
-      onchainWallets: []
+      }
     }
   },
   computed: {
@@ -302,42 +301,6 @@ window.PageEvents = {
       if (!value) return ''
       return value.length > 4 ? `${value.slice(0, 4)}...` : value
     },
-    async loadOnchainWallets() {
-      const wallet = _.findWhere(this.g.user.wallets, {
-        id: this.formDialog.data.wallet
-      })
-      if (!wallet) return
-      try {
-        const {data} = await LNbits.api.request(
-          'GET',
-          '/events/api/v1/events/onchain/status',
-          wallet.adminkey
-        )
-        this.onchainWallets = data.available ? data.wallets || [] : []
-      } catch {
-        this.onchainWallets = []
-      }
-    },
-    async confirmOnchainTicket(ticket) {
-      const wallet = _.findWhere(this.g.user.wallets, {id: ticket.wallet})
-      if (!wallet) return
-      try {
-        await LNbits.api.request(
-          'PUT',
-          `/events/api/v1/tickets/${ticket.id}/onchain-confirm`,
-          wallet.adminkey
-        )
-        Quasar.Notify.create({
-          type: 'positive',
-          message: 'Onchain payment confirmed.',
-          icon: null
-        })
-        await this.getTickets()
-        await this.getAllTickets()
-      } catch (error) {
-        LNbits.utils.notifyApiError(error)
-      }
-    },
     primaryTicketWave(data = this.formDialog.data) {
       if (!data.extra) data.extra = {}
       if (!data.extra.ticket_waves || data.extra.ticket_waves.length === 0) {
@@ -841,10 +804,6 @@ window.PageEvents = {
             min_tickets: 1,
             email_notifications: false,
             nostr_notifications: false,
-            onchain_enabled: false,
-            onchain_wallet_id: null,
-            onchain_zeroconf: false,
-            onchain_fasttrack: false,
             ticket_waves: [
               {
                 id: 'primary',
@@ -867,12 +826,6 @@ window.PageEvents = {
         }
       }
       this.formDialog.show = true
-      if (
-        this.formDialog.data.wallet &&
-        this.formDialog.data.extra?.onchain_enabled
-      ) {
-        this.loadOnchainWallets()
-      }
     },
     resetEventDialog() {
       this.formDialog.show = false
@@ -888,8 +841,6 @@ window.PageEvents = {
           min_tickets: 1,
           email_notifications: false,
           nostr_notifications: false,
-          onchain_enabled: false,
-          onchain_wallet_id: null,
           ticket_waves: [
             {
               id: 'primary',