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/lib/api.ts

@@ -577,6 +577,10 @@ export type ChatRoom = {
   members: number[];
   lastMessage: ChatMessage | null;
   lastReadId: number;
+  /** Messages up to this id were deleted by the viewer for themselves. */
+  clearedId?: number;
+  /** Every member's read mark by user id ("Seen"); absent from an older server. */
+  reads?: Record<string, number>;
   unread: number;
 };
 /** One account in Settings → Users (`GET /api/users`). */
@@ -1645,6 +1649,11 @@ export const api = {
         method: "POST",
         body: JSON.stringify({ upTo }),
       }),
+    /** Delete the chat for yourself only (a direct chat leaves your list). */
+    clear: (id: number) =>
+      request<{ hidden: boolean }>(`/chat/rooms/${id}/clear`, { method: "POST" }),
+    /** The group's creator deletes it for everyone. */
+    disband: (id: number) => request<{ ok: true }>(`/chat/rooms/${id}`, { method: "DELETE" }),
     /** Your own message only (403 otherwise). */
     deleteMessage: (roomId: number, messageId: number) =>
       request<{ ok: true }>(`/chat/rooms/${roomId}/messages/${messageId}`, { method: "DELETE" }),

client/src/lib/types.ts

@@ -1919,13 +1919,14 @@ export interface WSMessage {
    *  remote_data.updated → RemoteDataUpdatedPayload;
    *  lane_update → { lane?: Lane; removed?: number };
    *  chat_message → { roomId, message: ChatMessage, to }; chat_room → { roomId, to };
-   *  chat_deleted → { roomId, messageId, to }. */
+   *  chat_deleted → { roomId, messageId, to }; chat_read → { roomId, userId, lastReadId, to }. */
   type:
     | "session_created"
     | "session_updated"
     | "chat_message"
     | "chat_room"
     | "chat_deleted"
+    | "chat_read"
     | "agent_created"
     | "agent_updated"
     | "new_event"
@@ -1974,7 +1975,14 @@ export interface WSMessage {
     | LaneRuntimePayload
     | LaneHookResultPayload
     /** chat_message / chat_room (lib/api.ts ChatMessage). */
-    | { roomId: number; message?: unknown; messageId?: number; to?: number[] };
+    | {
+        roomId: number;
+        message?: unknown;
+        messageId?: number;
+        userId?: number;
+        lastReadId?: number;
+        to?: number[];
+      };
   /** ISO timestamp the server broadcast this message (not necessarily the
    *  same instant the underlying event occurred). */
   timestamp: string;