Implement DDS exercise, customer UI and local demo

backend/app/db/base.py

@@ -2,6 +2,7 @@
 
 from collections.abc import AsyncIterator
 
+from fastapi import HTTPException
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
 from sqlalchemy.orm import DeclarativeBase
 
@@ -45,5 +46,7 @@ def reset() -> None:
 
 async def get_session() -> AsyncIterator[AsyncSession]:
     """Зависимость FastAPI: сессия на запрос, коммит явный."""
+    if get_settings().demo_no_db:
+        raise HTTPException(status_code=503, detail="database_disabled_demo")
     async with get_sessionmaker()() as session:
         yield session

backend/app/db/repo.py

@@ -75,7 +75,7 @@ async def ensure_session(
         scenario_id=scenario_id,
         mode=mode,
         trainee_id=trainee.id if trainee else None,
-        group_id=group.id if group else None,
+        group_id=group.id if group else trainee.group_id if trainee else None,
         session_id=session_id,
     )