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/authentication.py

@@ -1,5 +1,6 @@
 from django.conf import settings
 from django.core import signing
+from django.core.cache import cache
 from rest_framework.authentication import BaseAuthentication
 from rest_framework.exceptions import AuthenticationFailed
 
@@ -7,6 +8,48 @@ from .models import User
 
 ACCESS_TOKEN_SALT = 'tadrisino-access-token'
 
+# فیلدهای غیرِ‌رابطه‌ایِ User که کش می‌شن؛ همه‌ی concrete fieldها تا بازسازیِ
+# instance برای save(update_fields=...) امن بمونه.
+_USER_CACHE_FIELDS = [f.attname for f in User._meta.concrete_fields]
+
+
+def _auth_user_cache_key(user_id: int) -> str:
+    return f'auth:user:{user_id}'
+
+
+def invalidate_user_cache(user_id: int) -> None:
+    cache.delete(_auth_user_cache_key(user_id))
+
+
+def _load_user_or_fail(user_id: int) -> User:
+    try:
+        return User.objects.get(pk=user_id)
+    except User.DoesNotExist:
+        raise AuthenticationFailed('کاربر پیدا نشد.')
+
+
+def _get_request_user(user_id: int) -> User:
+    """
+    ردیفِ کاربر برای request جاری. با AUTH_USER_CACHE_TTL>0 از کش خونده
+    می‌شه (یک SELECT کمتر در هر request authenticated). instanceِ بازسازی‌شده
+    کاملِ همه‌ی فیلدهاست، پس save(update_fields=[...]) روش امنه.
+    """
+    ttl = getattr(settings, 'AUTH_USER_CACHE_TTL', 0)
+    if not ttl:
+        return _load_user_or_fail(user_id)
+
+    key = _auth_user_cache_key(user_id)
+    data = cache.get(key)
+    if data is None:
+        user = _load_user_or_fail(user_id)
+        cache.set(key, {f: getattr(user, f) for f in _USER_CACHE_FIELDS}, ttl)
+        return user
+
+    user = User(**data)
+    user._state.adding = False
+    user._state.db = 'default'
+    return user
+
 
 def issue_access_token(user_id: int) -> str:
     """
@@ -48,13 +91,7 @@ class TadrisinoTokenAuthentication(BaseAuthentication):
 
         token = parts[1]
         user_id = decode_access_token(token)
-
-        try:
-            user = User.objects.get(pk=user_id)
-        except User.DoesNotExist:
-            raise AuthenticationFailed('کاربر پیدا نشد.')
-
-        return (user, token)
+        return (_get_request_user(user_id), token)
 
     def authenticate_header(self, request):
         return self.keyword