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.

docs/API.md

@@ -3503,18 +3503,20 @@ The pet's window has a rail of chats beside the chat with the pet: direct chats
 
 ```
 GET    /api/chat/people                         {people: [{id, username, displayName, avatarAt}]} — every enabled account but you
-GET    /api/chat/rooms                          {rooms} — yours, newest first: members, lastMessage, lastReadId, unread
+GET    /api/chat/rooms                          {rooms} — yours, newest first: members, lastMessage, lastReadId, clearedId, reads (every member's read mark — "Seen"), unread
 POST   /api/chat/rooms                          {kind: "dm", userId} → the pair's one direct chat (201 new / 200 existing); {kind: "group", name?, userIds} → 201
 PATCH  /api/chat/rooms/:id                      {name?, add?, remove?} — the group's creator only (403 EFORBIDDEN)
+DELETE /api/chat/rooms/:id                      Disband a group — its creator only, for every member, files included
+POST   /api/chat/rooms/:id/clear                Delete the chat for YOURSELF: {hidden, purged} — see below
 POST   /api/chat/rooms/:id/leave                Leave a group; the last one out deletes it and its files
 GET    /api/chat/rooms/:id/messages?before=&limit=   {messages (oldest first), hasMore} — 50 by default, ≤ 200
 POST   /api/chat/rooms/:id/messages             JSON {body} or multipart `body` + ≤ 10 `files` of ≤ 25 MB; body ≤ 8000 chars
 DELETE /api/chat/rooms/:id/messages/:messageId   Delete your OWN text message and its files (403 EFORBIDDEN otherwise, an admin included)
-POST   /api/chat/rooms/:id/read                 {upTo} — read mark, never moves back
+POST   /api/chat/rooms/:id/read                 {upTo} — read mark, never moves back nor past the newest message; a move is sent as `chat_read`
 GET    /api/chat/rooms/:id/files/:fileId        The file, members only, `sandbox` CSP + `nosniff`; PNG/JPEG/GIF/WebP inline, else attachment
 ```
 
-Files are stored under `<data dir>/chat/<room id>/<32 hex id>` — a name the server makes. Right-click menus in the window: a room (open, mark read, mute notifications — per browser —, copy name, group settings, leave), the rail's pet and **+** (mark all read), a message (copy text, reply, message the sender privately, delete your own), a file (open, download, copy name) and a group member (message privately, remove — the creator). A group change adds a `system` message (`created`, `renamed`, `left`, `added:<ids>`, `removed:<ids>`). Every change is sent over `/ws` as `chat_message` (`{roomId, message, to}`), `chat_room` (`{roomId, to}`) or `chat_deleted` (`{roomId, messageId, to}`) to exactly the user ids in `to` (`scope.exactAudience`), whatever their role.
+Files are stored under `<data dir>/chat/<room id>/<32 hex id>` — a name the server makes. Right-click menus in the window: a room (open, mark read, mute notifications — per browser —, copy name, group settings, leave), the rail's pet and **+** (mark all read), a message (copy text, reply, message the sender privately, delete your own), a file (open, download, copy name) and a group member (message privately, remove — the creator). **Deleting a chat** is per person: the messages so far vanish for the caller only (`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 — the messages up to the lowest `cleared_id` and their files on disk, and a direct chat both people deleted, whole (`purge`, also run when a member leaves or is removed). **Seen**: the room shows under your newest message "Seen" (direct chat), "Seen by …" or "Seen by everyone" (group), from `reads`. A group change adds a `system` message (`created`, `renamed`, `left`, `added:<ids>`, `removed:<ids>`). Every change is sent over `/ws` as `chat_message` (`{roomId, message, to}`), `chat_room` (`{roomId, to}`) or `chat_deleted` (`{roomId, messageId, to}`) or `chat_read` (`{roomId, userId, lastReadId, to}`) to exactly the user ids in `to` (`scope.exactAudience`), whatever their role.
 
 ### Session snapshots