Reorder Metrics

Persist metric display order and group membership after a drag-and-drop.

Body: {"groups": [{"category_id": 3, "metric_ids": [7, 2, 9]}, ...]}. Each entry lists a category's metrics in top-to-bottom order. Every listed metric is assigned category_id = the entry's category and position = its index — so one call covers both within-group reordering (send one entry) and moving a metric across groups (send the source and destination entries, each with its full new membership, so positions stay dense on both sides).

Integration-backed groups are read-only: a metric can neither be reordered into one nor moved out of one.