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.

models.py

@@ -98,14 +98,6 @@ class EventExtra(EventExtraBase):
     # `sync_event_ticket_waves`), which is why fork code that reads them
     # needs auditing — aiolabs/events#61.
     ticket_waves: list[TicketWave] = Field(default_factory=list)
-    # Upstream's SatsPay-backed on-chain config. Removed in the commit
-    # that follows this merge — we use native lnbits on-chain instead
-    # (aiolabs/events#41). Kept here so the removal is a legible diff
-    # against what upstream shipped rather than an invisible omission.
-    onchain_enabled: bool = False
-    onchain_wallet_id: str | None = None
-    onchain_zeroconf: bool = False
-    onchain_fasttrack: bool = False
 
 
 PublicEventExtra = EventExtraBase
@@ -256,9 +248,11 @@ class TicketExtra(BaseModel):
     email_notification_sent: bool = False
     nostr_notification_sent: bool = False
     refunded: bool = False
+    # On-chain vocabulary, populated once native lnbits on-chain lands
+    # (aiolabs/events#41). Upstream's field names, minus the SatsPay charge
+    # id — SatsPay is the implementation we are not adopting.
     onchain: bool = False
     onchain_address: str | None = None
-    satspay_charge_id: str | None = None
 
 
 class CreateTicket(BaseModel):
@@ -368,7 +362,6 @@ class TicketPaymentRequest(BaseModel):
     # My Tickets.
     ticket_ids: list[str] = Field(default_factory=list)
     onchain_amount_sat: int | None = None
-    satspay_charge_url: str | None = None
 
 
 class TicketFilters(FilterModel):