feat(bot): add async DB and FSM state handling

bot/services/users.py

@@ -0,0 +1,30 @@
+from aiogram.types import User as TelegramUser
+from sqlalchemy import select
+
+from bot.config import ADMIN_ID
+from bot.db.models import User
+from bot.db.session import async_session
+
+
+async def upsert_user_from_telegram(telegram_user: TelegramUser | None) -> User | None:
+    if telegram_user is None:
+        return None
+
+    async with async_session() as session:
+        result = await session.execute(
+            select(User).where(User.telegram_id == telegram_user.id)
+        )
+        user = result.scalar_one_or_none()
+
+        if user is None:
+            user = User(telegram_id=telegram_user.id)
+            session.add(user)
+
+        user.username = telegram_user.username
+        user.first_name = telegram_user.first_name
+        user.last_name = telegram_user.last_name
+        user.is_admin = telegram_user.id == ADMIN_ID
+
+        await session.commit()
+        await session.refresh(user)
+        return user