fix(payments): apply request timeout changes without restart

PAYMENT_REQUEST_TIMEOUT_SECONDS was read once in each provider's
__init__ and baked into the aiohttp session, so admin overrides
(applied in-process) only took effect after a container restart.
Providers now hand HttpClientMixin a timeout source callable; the
mixin builds the session with the current value and swaps in a fresh
session when the value changes, closing the replaced one only after
any in-flight request on it is bound by its own total timeout.

Also:
- check the Heleket payment-info success flag before reading the
  payload so a non-dict provider response cannot raise in the
  pending-payment reuse path
- add PAYMENT_REQUEST_TIMEOUT_SECONDS to the FreeKassa settings stub
  in test_security.py (fixes three tests broken by the new field)

backend/bot/payment_providers/severpay.py

@@ -138,7 +138,7 @@ class SeverPayService(HttpClientMixin):
         self.referral_service = referral_service
         self._default_return_url = default_return_url
 
-        self._init_http_client(total_timeout=self.settings.PAYMENT_REQUEST_TIMEOUT_SECONDS)
+        self._init_http_client(total_timeout=lambda: self.settings.PAYMENT_REQUEST_TIMEOUT_SECONDS)
 
         if not self.configured:
             logging.warning(