BUG-122: Remove channelID==0 bypass in deliverBroadcast that leaked
all channel-scoped broadcasts to unfocused clients. Clients must now
send channel_focus to receive channel events.
BUG-126: Reject edits and reactions on soft-deleted messages in
handleChatEdit and handleReaction.
BUG-108: Revoke all other sessions when a user changes their password
or enables/disables TOTP 2FA. Adds DeleteOtherSessions DB function.
7 new test cases covering all three fixes.