make streaks and the leaderboard live, and stop three AI chats from freezing the API

Found in a deeper pass over what the frontend's flow actually touches:

- Nothing in the app ever wrote StreakRecord: "days in a row" was 0 forever
  for every real user. Answers and lesson/island completions now record the
  day (Tehran time); a broken chain shows 0. A streak error never fails the
  answer itself.
- /api/leaderboard only changed when someone ran rebuild_leaderboard by
  hand. It is now computed live (60s cache); weekly counts island XP too.
- gunicorn ran 3 sync workers with a 30s timeout, so every "ask Pi" stream
  pinned a whole worker (~10s, GapGPT buffers the answer) and three at once
  queued the entire API. gunicorn.conf.py (read automatically from the
  working directory) switches to gthread, 8 threads, 120s timeout.
- Every /api/ response now says Cache-Control: private, no-store, so turning
  on caching in the ArvanCloud panel can never serve one student another's data.
- /api/user/summary named no active plan ever ("subscription app not
  implemented yet" had long stopped being true).

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

progress/leaderboard.py

@@ -0,0 +1,49 @@
+"""
+امتیازهای جدولِ رتبه‌بندی، مستقیم از فعالیتِ واقعی.
+
+تا ۲۰۲۶-۰۹-۲۴ `GET /api/leaderboard` فقط جدولِ LeaderboardEntry را می‌خواند که
+تنها با اجرای دستیِ `rebuild_leaderboard` پر می‌شد؛ XPی که کاربرِ تازه می‌گرفت
+هیچ‌وقت در جدول دیده نمی‌شد. حالا view همین تابع را صدا می‌زند و نتیجه را
+کوتاه‌مدت کش می‌کند، و دستورِ rebuild هم از همین تابع می‌خواند تا دو منبعِ
+عدد از هم جدا نیفتند.
+
+- `all_time`: همان `UserStats.xp` که در `/api/user/summary` هم دیده می‌شود.
+- `weekly`: XPی درس‌ها و جزیره‌هایی که در `days` روزِ اخیر تمام شده‌اند —
+  همان دو جایی که `UserStats.xp` زیاد می‌شود. `UserAnswer.xp_earned` عمداً
+  شمرده نمی‌شود چون به `UserStats.xp` اضافه نمی‌شود.
+"""
+from collections import defaultdict
+
+from django.db.models import Sum
+from django.utils import timezone
+
+PERIODS = ('weekly', 'all_time')
+
+
+def compute_scores(period, days=7, limit=50, include_zero=False):
+    """[(user_id, xp), ...] مرتب: XPی بیشتر جلوتر، در تساوی user_id کوچک‌تر.
+    پیش‌فرض فقط XPی مثبت؛ include_zero برای دستورِ rebuild است."""
+    if period == 'all_time':
+        from progress.models import UserStats
+        qs = UserStats.objects.all() if include_zero else UserStats.objects.filter(xp__gt=0)
+        rows = list(
+            qs.order_by('-xp', 'user_id')
+            .values_list('user_id', 'xp')[:limit]
+        )
+        return [(uid, xp) for uid, xp in rows]
+
+    if period != 'weekly':
+        return []
+
+    from content.models import UserLessonCompletion, UserNodeCompletion
+    since = timezone.now() - timezone.timedelta(days=days)
+    totals = defaultdict(int)
+    for model in (UserLessonCompletion, UserNodeCompletion):
+        for row in (
+            model.objects.filter(created_at__gte=since)
+            .values('user_id').annotate(total=Sum('xp_earned'))
+        ):
+            totals[row['user_id']] += row['total'] or 0
+    scores = [(uid, xp) for uid, xp in totals.items() if include_zero or xp > 0]
+    scores.sort(key=lambda p: (-p[1], p[0]))
+    return scores[:limit]