Bulk Update Items By Ids

Update fields applicable to a bulk operation across a group.

Mirrors the bulk-applicable subset of :class:ItemUpdate. rd_percentage and description are intentionally excluded — they're meaningful per-item, not per-group.

activity_id uses "explicit null = Ungrouped" semantics like its :class:ItemUpdate twin — keyed on model_fields_set in the handler.

subgroup_key moves every targeted item into a (possibly new) subgroup within its existing category. It's only sent by the by-ids path (the categorised multi-select "Move to / New subgroup" action); unlike activity_id there's no "explicit null = clear" semantic — a move always names a non-empty target, validated in the handler.