Return every validation plot of an element at full extent, in one response.

What the first paint of a validation section needs: one request for all objectives rather than two per objective. Each plot is decimated to at most max_points per trace, lowered evenly across the batch where the whole body would otherwise exceed the batch payload cap; the served budget is returned as max_points and each plot reports whether it is complete at that budget. Zoom into one plot through GET /pipelines/elements/{element_id}/plot_data.