optional short-TTL cache for the auth user row

TadrisinoTokenAuthentication ran one SELECT on accounts_user for every
authenticated request. Off by default (AUTH_USER_CACHE_TTL=0) so behaviour
is unchanged until a deployment opts in.

accounts/authentication.py
  - _get_request_user(): when AUTH_USER_CACHE_TTL>0, cache all concrete
    field values under auth:user:<id> and rebuild a full User instance
    (so save(update_fields=[...]) stays safe); otherwise load from DB as
    before. invalidate_user_cache() / _load_user_or_fail() helpers.
accounts/receivers.py (new)
  - post_save/post_delete on User -> invalidate_user_cache(pk); covers
    every in-app write path (SetUserGrade, login, refresh, last_activity).
accounts/apps.py
  - AccountsConfig.ready() imports receivers.
accounts/tests.py
  - AuthUserRowCacheTests: disabled -> loads every request; enabled ->
    loads once then cache; User.save() invalidates; SetUserGradeView still
    persists and the next request sees the new grade.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

accounts/tests.py

@@ -1430,3 +1430,66 @@ class SupervisorGroupAccessTests(TestCase):
         self.client.force_login(non_staff_supervisor)
         response = self.client.get(reverse('admin:accounts_user_changelist'))
         self.assertEqual(response.status_code, 302)
+
+
+class AuthUserRowCacheTests(TestCase):
+    """کشِ کوتاهِ ردیفِ کاربر در TadrisinoTokenAuthentication."""
+
+    def setUp(self):
+        cache.clear()
+        self.client = APIClient()
+        self.user = User.objects.create(device_id='dev-authcache', is_guest=True, name='n')
+        self.token = issue_access_token(self.user.id)
+        # هر endpointِ authenticated؛ /api/user/hearts سبک است.
+        self.url = '/api/user/hearts'
+
+    def _hit(self):
+        return self.client.get(self.url, HTTP_AUTHORIZATION=f'Bearer {self.token}')
+
+    @override_settings(AUTH_USER_CACHE_TTL=0)
+    def test_disabled_by_default_loads_every_request(self):
+        with patch('accounts.authentication._load_user_or_fail', wraps=_orig_loader()) as m:
+            self._hit(); self._hit()
+        self.assertEqual(m.call_count, 2)
+
+    @override_settings(AUTH_USER_CACHE_TTL=30)
+    def test_enabled_caches_after_first_load(self):
+        with patch('accounts.authentication._load_user_or_fail', wraps=_orig_loader()) as m:
+            self.assertEqual(self._hit().status_code, 200)
+            self.assertEqual(self._hit().status_code, 200)
+        self.assertEqual(m.call_count, 1)
+
+    @override_settings(AUTH_USER_CACHE_TTL=30)
+    def test_user_save_invalidates_cache(self):
+        self._hit()  # cache populated
+        with patch('accounts.authentication._load_user_or_fail', wraps=_orig_loader()) as m:
+            self.user.name = 'changed'
+            self.user.save(update_fields=['name'])  # post_save → invalidate
+            self._hit()
+        self.assertEqual(m.call_count, 1)
+
+    @override_settings(AUTH_USER_CACHE_TTL=30)
+    def test_set_grade_still_works_with_cache_on(self):
+        r = self.client.post(
+            '/api/user/grade', {'gradeId': 3}, format='json',
+            HTTP_AUTHORIZATION=f'Bearer {self.token}')
+        self.assertEqual(r.status_code, 200)
+        self.user.refresh_from_db()
+        self.assertEqual(self.user.grade_id, 3)
+        # درخواستِ بعدی باید گریدِ تازه را ببیند (کش پس از save باطل شده).
+        prof = self.client.get(
+            '/api/user/profile', HTTP_AUTHORIZATION=f'Bearer {self.token}')
+        self.assertEqual(prof.data['gradeId'], 3)
+
+
+def _orig_loader():
+    """نسخه‌ی اصلیِ لودر برای wraps کردنِ Mock (بدونِ recursion)."""
+    from accounts.models import User as _U
+    from rest_framework.exceptions import AuthenticationFailed
+
+    def _load(user_id):
+        try:
+            return _U.objects.get(pk=user_id)
+        except _U.DoesNotExist:
+            raise AuthenticationFailed('کاربر پیدا نشد.')
+    return _load