feat: SAC 0.7.4 sidebar system stats block with UI toggle

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/models/ui_settings.py

@@ -0,0 +1,15 @@
+from sqlalchemy import Boolean
+from sqlalchemy.orm import Mapped, mapped_column
+
+from app.database import Base
+
+UI_SETTINGS_ROW_ID = 1
+
+
+class UiSettings(Base):
+    """Singleton: глобальные настройки интерфейса SAC."""
+
+    __tablename__ = "ui_settings"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    show_sidebar_system_stats: Mapped[bool] = mapped_column(Boolean, default=True, nullable=False)