feat(chat): delete a chat, disband a group, Seen — and purge what everyone deleted

Delete chat is the caller's alone: the history so far vanishes for them
(chat_members.cleared_id) and a direct chat leaves their list (hidden) until a
new message arrives; the others keep theirs. What EVERY member has deleted is
deleted for real (purge): messages up to the lowest cleared_id and their files
on disk, and a direct chat both people deleted, whole. Purge also runs when a
member leaves or is removed. A group's creator disbands it for everyone
(DELETE /api/chat/rooms/:id).

Seen: rooms carry every member's read mark (reads); a mark that moves is told
to the members (chat_read, exactAudience) and never passes the newest
message. Under your newest message: Seen / Seen by … / Seen by everyone.
A room open on a tab you come back to is marked read then.

client/src/components/Sagi/Sagi.tsx

@@ -453,6 +453,8 @@ function SagiBody() {
               <NewChat chat={people} onOpen={setPane} onClose={() => setOpen(false)} />
             ) : people.enabled && room ? (
               <ChatRoomView
+                // Cleared for me: read the (now shorter) history again.
+                key={`${room.id}:${room.clearedId ?? 0}`}
                 chat={people}
                 room={room}
                 onClose={() => setOpen(false)}