give the leaderboard and notifications a real producer

Three gaps found while seeding test data for the frontend, all of the same
kind: a contract that looks complete from the outside and is empty underneath.

Nothing in the codebase ever wrote a LeaderboardEntry. Not a signal, not a
service — only the admin form and seed_test_data. So /api/leaderboard returned
`entries: []` forever on any real server, and any row put there was a frozen
number that never tracked activity. rebuild_leaderboard now derives both
periods from real XP: all_time from UserStats.xp, weekly from a sum over
content.UserLessonCompletion in a window. It deliberately does not count
UserAnswer.xp_earned — that field is written but never added to UserStats, so
summing both would report roughly double. deploy.sh runs it every release;
a cron is the real answer once traffic is real, and the script says so.

'lesson_completed' was declared in Notification.TYPE_CHOICES but nothing
called notify() with it, so finishing a lesson produced no notification at
all. LessonCompleteView now emits one, on first completion only (the same
condition that grants xp/coins), inside the completion transaction so a lost
notification cannot leave a silent half-done state.

The notification list serializer returned only id/type/title/isRead. body and
created_at were on the model from the start but never exposed, so the frontend
had neither the text of a notification nor its time. Both added; the change is
purely additive, so the existing client keeps working.

472 tests green, up from 458. makemigrations --check, check and spectacular
are all clean.

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

notifications/serializers.py

@@ -6,10 +6,15 @@ from .models import Notification
 class NotificationItemSerializer(serializers.ModelSerializer):
     id = serializers.IntegerField()
     isRead = serializers.BooleanField(source='is_read')
+    # body و createdAt از ابتدا روی مدل بودند ولی در پاسخ نبودند، یعنی فرانت
+    # نه متنِ اعلان را داشت نه زمانش — و هر لیستِ اعلانی به هر دو نیاز دارد.
+    # هر دو فقط «اضافه» می‌شوند؛ هیچ فیلدی حذف یا تغییرِ نام نداده، پس
+    # کلاینتِ فعلی نمی‌شکند.
+    createdAt = serializers.DateTimeField(source='created_at', read_only=True)
 
     class Meta:
         model = Notification
-        fields = ['id', 'type', 'title', 'isRead']
+        fields = ['id', 'type', 'title', 'body', 'isRead', 'createdAt']
 
 
 class NotificationListResponseSerializer(serializers.Serializer):