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/routes/chat.js

@@ -118,6 +118,24 @@ router.post(
   })
 );
 
+// Delete the chat for the caller only (their history; a direct chat leaves their list).
+router.post(
+  "/rooms/:id/clear",
+  sameOriginGuard,
+  handle((me, req, res) => res.json(chat.clearChat(me, req.params.id)))
+);
+
+// The group's creator deletes it for everyone.
+router.delete(
+  "/rooms/:id",
+  sameOriginGuard,
+  handle((me, req, res) => {
+    const { notify } = chat.disbandGroup(me, req.params.id);
+    tell("chat_room", { roomId: Number(req.params.id) }, notify);
+    res.json({ ok: true });
+  })
+);
+
 router.get(
   "/rooms/:id/messages",
   handle((me, req, res) =>
@@ -153,7 +171,12 @@ router.delete(
 router.post(
   "/rooms/:id/read",
   sameOriginGuard,
-  handle((me, req, res) => res.json(chat.markRead(me, req.params.id, req.body?.upTo)))
+  handle((me, req, res) => {
+    const { lastReadId, moved, notify } = chat.markRead(me, req.params.id, req.body?.upTo);
+    // "Seen": the other members' views move their read marks.
+    if (moved) tell("chat_read", { roomId: Number(req.params.id), userId: me, lastReadId }, notify);
+    res.json({ lastReadId });
+  })
 );
 
 router.get(