Merge groups (v2)

Merge one or more source groups into the group identified by groupId. The destination keeps its ID, receives the final name, emoji, and de-duplicated union of contact memberships, while selected source groups are deleted. Direct subgroups are reparented under the destination and source-linked saved views are retargeted. Requires Idempotency-Key. A merge whose combined source memberships exceed the synchronous limit is refused up front with a 400 naming the limit; split it into smaller merges. A 500 timeout means nothing was committed and the same request is safe to retry, while a 500 merge_committed_read_failed means the merge DID commit and only its response could not be built — refetch instead of retrying.