Version control for dashboards

**What the feature is (as Title)**
Users can track, manage, and revert changes to dashboards through a built-in version history, similar to version control in collaborative document editors.

**Value proposition**
Dashboards evolve over time as teams add panels, adjust queries, and change layouts. Without version control, a single accidental edit or misconfiguration can break a dashboard with no easy way to undo it. This feature gives users confidence to iterate on dashboards knowing they can always review what changed, who changed it, and roll back to a previous state.

**Expected outcome**
Users can view a full history of changes made to a dashboard, including who made each change and when.
Any previous version can be restored with a single action.
Users can compare versions side by side to understand what changed.

**Optional: Key user stories / use cases**