Wire the bitwise permission engine end-to-end:
- Add sendToChannel() to ConnectionManager, filtering WS recipients by VIEW_CHANNEL
- Convert 6 channel-scoped events (messages, typing, reactions) from sendToServer to sendToChannel
- Add broadcastOverrideChange() to push channel_updated/channel_deleted per-user on override mutations
- Bridge legacy server_members.role TEXT to member_roles junction table on PATCH
- Add pushReadyPayload() to force re-sync frontend store after role changes
- Filter channels by VIEW_CHANNEL in GET /api/servers/:id to prevent initial load data leak
- Pre-compute viewers before CASCADE delete on channel_deleted
- Fix frontend channel event handlers to upsert/cleanup channelToServerMap and channelPermissions
- Add ChannelSettingsModal with Private Channel toggle and gear icon in ChannelSidebar