Patch workspace-global defaults. Only the fields passed are changed.

handoff_template (7855e580): a blank/whitespace-only value here is treated as "field left untouched," never as an explicit clear-to-server-default. This dashboard endpoint is the one an ordinary Save Defaults click hits — e.g. while the settings form's own GET is still in flight, its template textarea renders empty — and that must never be able to silently wipe out a workspace's real, already-configured non-empty handoff_template. A genuine explicit clear (empty string forwarded verbatim) is still available through the MCP update_workspace_settings tool, which calls db.update_workspace_settings directly and does not go through this route.