move the API's own address to mathapi.tadrisino2.ir

The API moved to mathapi.tadrisino2.ir on 2026-09-26 (nginx server_name
changed at 23:22) and api.math.tadrisino2.ir no longer resolves. The code
default for PUBLIC_BASE_URL and the docs' curl examples still named the dead
host, so every paymentUrl, the Zarinpal callback and the Bazaar redirect the
backend builds pointed nowhere. Historical status notes in CLAUDE.md keep the
old name on purpose. config/tests.py now reads the API host from
PUBLIC_BASE_URL instead of a literal. 624 tests green.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

config/tests.py

@@ -2,6 +2,7 @@ import importlib
 import json
 import logging
 import os
+from urllib.parse import urlsplit
 from unittest.mock import patch
 
 from django.conf import settings
@@ -318,7 +319,8 @@ class ServerEnvDefaultsTests(SimpleTestCase):
     اینکه بدونِ هیچ متغیرِ محیطی‌ای مقدارِ کارآمد دارند.
     """
 
-    HOST = 'api.math.tadrisino2.ir'
+    # دامنه‌ی API از PUBLIC_BASE_URL می‌آید، نه از یک رشته‌ی ثابت در تست.
+    HOST = urlsplit(settings.PUBLIC_BASE_URL).hostname
 
     def test_the_api_domain_is_allowed_without_any_env_var(self):
         self.assertIn(self.HOST, settings.DEFAULT_ALLOWED_HOSTS)
@@ -341,7 +343,7 @@ class ServerEnvDefaultsTests(SimpleTestCase):
         """کلید در .env هست ولی خالی — os.environ.get پیش‌فرض نمی‌دهد."""
         with patch.dict(os.environ, {'PUBLIC_BASE_URL': ''}):
             value = (os.environ.get('PUBLIC_BASE_URL', '').strip()
-                     or 'https://api.math.tadrisino2.ir')
+                     or 'https://mathapi.tadrisino2.ir')
         self.assertTrue(value.startswith('https://'))
 
     def test_admin_login_origin_is_trusted_without_any_env_var(self):