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>

content/services.py

@@ -15,11 +15,11 @@ from .models import Lesson, Node, NodeProgress, UserLessonCompletion, UserNodeCo
 
 
 def _get_or_create_progress(user_id, node):
-    """get_or_create ایجاد می‌کند، سپس ردیف با select_for_update دوباره خوانده و قفل می‌شود
-    تا خواندن/تغییر/ذخیره در برابر concurrent update ایمن باشد. باید داخل transaction.atomic صدا زده شود."""
-    NodeProgress.objects.get_or_create(user_id=user_id, node=node)
-    progress = NodeProgress.objects.select_for_update().get(user_id=user_id, node=node)
-    return progress, False
+    """ردیف را با قفل (select_for_update) می‌گیرد یا می‌سازد، در یک کوئری برای حالتِ معمول (ردیف هست).
+    دو ساختِ هم‌زمان به قیدِ یکتاییِ (user_id, node) می‌خورند و get_or_create دومی را دوباره می‌خواند.
+    باید داخلِ transaction.atomic صدا زده شود."""
+    progress, created = NodeProgress.objects.select_for_update().get_or_create(user_id=user_id, node=node)
+    return progress, created
 
 
 def _emit_if_changed(progress, old_status, event, user_id):