Adds POST /api/dm/space-invite which fetches a space-invite snapshot
server-to-server from the space's home instance, ensures a 1-on-1 DM
between the caller and a friend, and posts a type='system' message
carrying SpaceInviteSystemPayload. Snapshot is never trusted from the
client. Rate-limited 30/60s per caller. Federation relay queued when the
recipient is on a remote instance (system message type forwarded by
Tasks 1-3).
Adds an `ensureOneOnOneDmChannel` helper that mirrors the dedup-or-create
behavior of the existing POST /api/dm handler — including federatedId
computation and the dm_channel_created notification payload — without
modifying that handler. Duplication is intentional; consolidation is a
separate follow-up.
Jannis Braun
5cb0aa9d66