V133: Dashboards table.

Backs the phoenix_kit_dashboards plugin module. A dashboard is a page of placed widgets; its layout is stored as a JSONB list of widget instances ([{id, widget_key, x, y, w, h, settings}]), read and written whole.

Scopes:

• personal — owner_user_uuid set; private to that user.
• system — owner_user_uuid NULL; visible to everyone.
• role — role_uuid set; visible to members of that role.

All statements are idempotent (IF NOT EXISTS), safe to re-run.