Merge every item in source into target. Each moved item picks up target.category + target.subgroup_key and is flagged manual_override=True.

Linked refunds are excluded from the sweep (derived rows) and instead follow their original via the mirror — in both directions: the mirror here re-flows the merged originals onto their refunds, and reverse_subgroup_merge mirrors again on restore so the refunds come back out of the target too.

Records an :class:AURDExpenditureSubgroupMergeEvent capturing each moved item's pre-merge tuple so an admin can later reverse the merge.