C-1: messaging endpoints now enforce channel participation (public
channels open; private require channel_members). authorUserId is
forced to the authenticated user (no more author spoofing); edit/
delete require message-author ownership; history read gated too.
C-2: PUT /commands body strictly validated + size-capped via
SyncCommandsDto (kills catalog poisoning / DoS). Optional
FABRIC_BACKEND_GUILD_COMMANDS_SYNC_KEY restricts the write to the
plugin when set; never weaker than before when unset.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>