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>

content/views.py

@@ -519,6 +519,25 @@ class LessonCompleteView(APIView):
                 ).values('lesson_id').distinct().count()
                 stats.progress_percent = round(min(completed_lessons / total_lessons, 1.0) * 100, 2)
                 stats.save(update_fields=['xp', 'coins', 'progress_percent'])
+
+                # اعلانِ درسِ تکمیل‌شده. تا پیش از این، نوعِ 'lesson_completed'
+                # در Notification.TYPE_CHOICES تعریف شده بود ولی هیچ‌جای کد
+                # آن را نمی‌ساخت — یعنی کلِ مسیرِ کوییز هیچ اعلانی تولید
+                # نمی‌کرد. فقط بارِ اول (همان شرطی که xp/coins می‌دهد)؛
+                # تکرارِ درس اعلانِ تازه نمی‌سازد.
+                #
+                # عمداً داخلِ همین transaction است: اگر نوشتنِ اعلان شکست
+                # بخورد، تکمیلِ درس هم برنمی‌گردد — هم‌الگو با
+                # campaigns.services که اعلان و وضعیت را یک واحد می‌داند.
+                if is_first_completion:
+                    from notifications.services import notify
+                    notify(
+                        user_id=user_id,
+                        type='lesson_completed',
+                        title='درس را کامل کردی',
+                        body=f'«{lesson.title}» را با {stars} ستاره تمام کردی و {xp_earned} امتیاز گرفتی.',
+                        related_id=lesson.id,
+                    )
         except IntegrityError:
             # قید یکتاییِ سطح DB جلوی درخواست‌های هم‌زمان با همان کلید را می‌گیرد؛
             # نتیجه‌ی completion رقیب idempotent برگردانده می‌شود.