Task A: REST API endpoints in api/dm_handler.go — POST /api/v1/dms
(create/get DM channel), GET /api/v1/dms (list open DMs), DELETE
/api/v1/dms/{channelId} (close DM). Routes registered in router.go.
Task B: WebSocket DM routing — handleChatSend, handleChatEdit,
handleChatDelete, and handleReaction all check channel type and use
participant-based auth for DM channels instead of role permissions.
DM messages delivered via SendToUser to each participant (bypassing
channel-subscription model). Auto-reopens DM for recipient on new
message with dm_channel_open event. New broadcastToDMParticipants
helper. New WS event types: dm_channel_open, dm_channel_close.
Task C: Ready payload includes dm_channels from GetUserDMChannels.
Also adds GetDMParticipantIDs helper to db/dm_queries.go.
