feat: SAC 0.7.4 sidebar system stats block with UI toggle

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

backend/app/services/ui_settings.py

@@ -0,0 +1,30 @@
+from dataclasses import dataclass
+
+from sqlalchemy.orm import Session
+
+from app.models.ui_settings import UI_SETTINGS_ROW_ID, UiSettings
+
+
+@dataclass(frozen=True)
+class UiSettingsConfig:
+    show_sidebar_system_stats: bool
+    source: str = "db"
+
+
+def get_effective_ui_settings(db: Session) -> UiSettingsConfig:
+    row = db.get(UiSettings, UI_SETTINGS_ROW_ID)
+    if row is None:
+        return UiSettingsConfig(show_sidebar_system_stats=True, source="default")
+    return UiSettingsConfig(show_sidebar_system_stats=bool(row.show_sidebar_system_stats), source="db")
+
+
+def upsert_ui_settings(db: Session, *, show_sidebar_system_stats: bool) -> UiSettingsConfig:
+    row = db.get(UiSettings, UI_SETTINGS_ROW_ID)
+    if row is None:
+        row = UiSettings(id=UI_SETTINGS_ROW_ID, show_sidebar_system_stats=show_sidebar_system_stats)
+        db.add(row)
+    else:
+        row.show_sidebar_system_stats = show_sidebar_system_stats
+    db.commit()
+    db.refresh(row)
+    return get_effective_ui_settings(db)