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.

services.py

@@ -9,9 +9,7 @@ from email.mime.multipart import MIMEMultipart
 from email.mime.text import MIMEText
 from email.utils import formataddr, formatdate, make_msgid
 from html import escape
-from typing import Any
 
-import httpx
 from lnbits.core.models.users import UserNotifications
 from lnbits.core.services.nostr import send_nostr_dm
 from lnbits.core.services.notifications import send_user_notification
@@ -45,59 +43,6 @@ from .qr import (
     ticket_card_filename,
 )
 
-
-async def fetch_watchonly_config(api_key: str) -> dict[str, Any]:
-    async with httpx.AsyncClient() as client:
-        resp = await client.get(
-            url=f"http://{settings.host}:{settings.port}/watchonly/api/v1/config",
-            headers={"X-API-KEY": api_key},
-        )
-        resp.raise_for_status()
-        return resp.json()
-
-
-async def fetch_watchonly_wallets(api_key: str, network: str) -> list[dict[str, Any]]:
-    async with httpx.AsyncClient() as client:
-        resp = await client.get(
-            url=f"http://{settings.host}:{settings.port}/watchonly/api/v1/wallet",
-            headers={"X-API-KEY": api_key},
-            params={"network": network},
-        )
-        resp.raise_for_status()
-        return resp.json()
-
-
-async def fetch_watchonly_wallet(api_key: str, wallet_id: str) -> dict[str, Any]:
-    async with httpx.AsyncClient() as client:
-        resp = await client.get(
-            url=f"http://{settings.host}:{settings.port}/watchonly/api/v1/wallet/{wallet_id}",
-            headers={"X-API-KEY": api_key},
-        )
-        resp.raise_for_status()
-        return resp.json()
-
-
-async def get_satspay_charge(api_key: str, charge_id: str) -> dict[str, Any]:
-    async with httpx.AsyncClient() as client:
-        resp = await client.get(
-            url=f"http://{settings.host}:{settings.port}/satspay/api/v1/charge/{charge_id}",
-            headers={"X-API-KEY": api_key},
-        )
-        resp.raise_for_status()
-        return resp.json()
-
-
-async def create_satspay_charge(api_key: str, data: dict) -> dict[str, Any]:
-    async with httpx.AsyncClient() as client:
-        resp = await client.post(
-            url=f"http://{settings.host}:{settings.port}/satspay/api/v1/charge",
-            headers={"X-API-KEY": api_key},
-            json=data,
-        )
-        resp.raise_for_status()
-        return resp.json()
-
-
 DEFAULT_NOSTR_RELAYS = [
     "wss://relay.damus.io",
     "wss://relay.primal.net",