feat: persist system settings and refine admin layouts

backend/app/models/system_setting.py

@@ -0,0 +1,19 @@
+"""Persistent system settings model."""
+
+from sqlalchemy import JSON, Column, DateTime, Integer, String, UniqueConstraint
+from sqlalchemy.sql import func
+
+from app.db.session import Base
+
+
+class SystemSetting(Base):
+    __tablename__ = "system_settings"
+    __table_args__ = (UniqueConstraint("category", name="uq_system_settings_category"),)
+
+    id = Column(Integer, primary_key=True, autoincrement=True)
+    category = Column(String(50), nullable=False)
+    payload = Column(JSON, nullable=False, default={})
+    updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())
+
+    def __repr__(self):
+        return f"<SystemSetting {self.category}>"