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/config/settings.py

@@ -329,7 +329,7 @@ class Settings(BaseSettings):
     STARS_ADMIN_ONLY_ENABLED: bool = Field(default=False)
     PAYMENT_METHODS_ORDER: Optional[str] = Field(
         default=None,
-        description="Comma-separated list of payment methods to show (e.g., severpay,wata,freekassa,yookassa,platega,stars,cryptopay,heleket,paykilla)",  # noqa: E501
+        description="Comma-separated list of payment methods to show (e.g., severpay,wata,freekassa,yookassa,platega,stars,cryptopay,heleket,paykilla,lava)",  # noqa: E501
     )
     SUBSCRIPTION_PURCHASE_DESCRIPTION_ENABLED: bool = Field(
         default=True,
@@ -1056,6 +1056,7 @@ class Settings(BaseSettings):
             "cryptopay",
             "heleket",
             "paykilla",
+            "lava",
         ]
         # Make sure default_order itself includes every registered spec.
         for sid in spec_ids: