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/db.js

@@ -1952,6 +1952,15 @@ db.exec(`
   );
   CREATE INDEX IF NOT EXISTS idx_chat_files_message ON chat_files(message_id);
 `);
+// A member's own "delete this chat": messages up to cleared_id are gone for
+// them (not for anyone else), and a hidden direct chat leaves their list until
+// a new message arrives. Additive.
+try {
+  db.prepare("SELECT cleared_id, hidden FROM chat_members LIMIT 1").get();
+} catch {
+  db.prepare("ALTER TABLE chat_members ADD COLUMN cleared_id INTEGER NOT NULL DEFAULT 0").run();
+  db.prepare("ALTER TABLE chat_members ADD COLUMN hidden INTEGER NOT NULL DEFAULT 0").run();
+}
 
 // An account's own agent instructions, put before every workspace's in each
 // pane it owns (server/lib/lane-sources.js, issue #67). Additive.