Add Chat Participants Batch

Add several participants in one action.

TOS-2938: a multi-select "add people" action used to call the singular add endpoint once per person, which produced one participant_added pill per person. This endpoint emits exactly one combined system pill for the whole batch.

Same caller-active + per-target connection gate as the singular endpoint, checked for every target before any participant is added.

TOS-XXXX: a target that is already an active participant is now SKIPPED rather than failing the whole batch. The "Add people" modal filters only on the client-side participant list it last loaded, so two callers opening it concurrently can both select someone the other just added; the previous behavior raised a bare, nameless 409 mid-loop and discarded the earlier adds in the same batch via session rollback. Already-active targets are returned in skipped_already_active so the UI can say "already in the room" per person.