Spike readiness: AI gets its own slice of threads, cheaper hot paths, classroom-friendly login

- /api/ai/ask: at most AI_MAX_CONCURRENT_PER_WORKER (12 of 32 threads) answers per worker;
  beyond that 503 assistant_busy + Retry-After before any quota is reserved. Without it a
  burst of questions (3-40 s each) took every thread and the map and quizzes queued behind.
  The slot is held until the stream is actually finished or closed.
- Answer submission 21 -> 14 queries, 15.4 -> 11.0 ms: one query for all choices, locked
  get_or_create for node progress and streak, and today's streak recorded once per day
  via the cache instead of a transaction per answer.
- /api/home 12.4 -> 8.6 ms: the user's completions are read by user_id instead of an IN list
  of ~600 lesson ids; the dict is returned directly (serializer stays the Swagger contract,
  HomeMatchesItsSerializerTests keeps them equal).
- otp_login per-IP 30 -> 300/hour: a class of 30 on one school Wi-Fi filled it; guessing is
  bounded per code and per phone.
- /api/health aiConcurrency.

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

progress/services.py

@@ -20,8 +20,7 @@ def record_activity(user_id, today=None):
     """امروز را روزِ فعال ثبت می‌کند و استریک را جلو می‌برد. idempotent در یک روز."""
     today = today or timezone.localdate()
     with transaction.atomic():
-        StreakRecord.objects.get_or_create(user_id=user_id)
-        streak = StreakRecord.objects.select_for_update().get(user_id=user_id)
+        streak, _ = StreakRecord.objects.select_for_update().get_or_create(user_id=user_id)
 
         last = (
             ActiveDay.objects.filter(streak=streak)