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.

server/lib/scope.js

@@ -347,7 +347,7 @@ function exactAudience(type, data) {
   const d = data && typeof data === "object" ? data : {};
   // Chat (server/lib/chat.js): only the room's members, whatever their role —
   // an admin reads no one else's chat. No list = nobody.
-  if (type === "chat_message" || type === "chat_room" || type === "chat_deleted") {
+  if (["chat_message", "chat_room", "chat_deleted", "chat_read"].includes(type)) {
     return new Set(Array.isArray(d.to) ? d.to : []);
   }
   if (type === "cc_config_changed" && d.ownerId != null) return new Set([d.ownerId]);