feat(payments): add LAVA Business payment provider

Add a new provider module for LAVA Business (api.lava.ru):

- invoice creation via POST /business/invoice/create signed with
  HMAC-SHA256 over the raw request body in the Signature header
- webhook handling with Authorization-header signature verification
  that accepts both raw-body and sorted-keys JSON canonicalizations
  (legacy PHP SDK shops sign the latter), plus amount cross-check
  before finalizing a successful payment
- pending invoice reuse through /business/invoice/status
- Telegram pay_lava callback flow and Web App payment creation
- admin settings manifest fields, presentation overrides, and
  RUB-only currency support declared on the provider SPEC

Register the SPEC in the provider registry and wire the provider into
the payment method order, success-email labels, and locale override
prefixes.

backend/bot/payment_providers/registry.py

@@ -2,7 +2,18 @@ from __future__ import annotations
 
 from typing import Any, Dict, Iterable, List, Mapping, Optional
 
-from . import cryptopay, freekassa, heleket, paykilla, platega, severpay, stars, wata, yookassa
+from . import (
+    cryptopay,
+    freekassa,
+    heleket,
+    lava,
+    paykilla,
+    platega,
+    severpay,
+    stars,
+    wata,
+    yookassa,
+)
 from .base import (
     PaymentProviderPresentation,
     PaymentProviderSpec,
@@ -22,6 +33,7 @@ PAYMENT_PROVIDER_SPECS: tuple[PaymentProviderSpec, ...] = (
     cryptopay.SPEC,
     heleket.SPEC,
     paykilla.SPEC,
+    lava.SPEC,
 )