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/openapi-extra/chat.js

@@ -59,7 +59,16 @@ const schemas = {
       members: { type: "array", items: { type: "integer" } },
       lastMessage: { allOf: [{ $ref: "#/components/schemas/ChatMessage" }], nullable: true },
       lastReadId: { type: "integer" },
+      clearedId: {
+        type: "integer",
+        description: "Messages up to this id were deleted by the caller for themselves.",
+      },
       unread: { type: "integer", description: "Messages of others after `lastReadId`." },
+      reads: {
+        type: "object",
+        additionalProperties: { type: "integer" },
+        description: "Every member's read mark by user id — what the room shows as Seen.",
+      },
     },
   },
 };
@@ -145,6 +154,20 @@ const paths = {
     },
   },
   "/api/chat/rooms/{id}": {
+    delete: {
+      tags: ["Chat"],
+      summary: "Disband a group",
+      description:
+        "The group's creator only (`403 EFORBIDDEN`): deletes it for every member, its files included. Broadcasts `chat_room`.",
+      parameters: [idParam],
+      responses: {
+        200: {
+          description: "Disbanded",
+          ...json({ type: "object", properties: { ok: { type: "boolean" } } }),
+        },
+        ...fail([400, 403, 404]),
+      },
+    },
     patch: {
       tags: ["Chat"],
       summary: "Rename a group, add or remove members",
@@ -164,6 +187,31 @@ const paths = {
       responses: { 200: { description: "Updated", ...roomResp }, ...fail([400, 403, 404]) },
     },
   },
+  "/api/chat/rooms/{id}/clear": {
+    post: {
+      tags: ["Chat"],
+      summary: "Delete a chat for yourself",
+      description:
+        "Every message so far is gone for the caller only (`clearedId`); a direct chat also leaves their list until a new message arrives. Nobody else's copy changes — but what EVERY member has deleted is deleted for real (messages, files, and a direct chat both people deleted, whole).",
+      parameters: [idParam],
+      responses: {
+        200: {
+          description: "Cleared",
+          ...json({
+            type: "object",
+            properties: {
+              hidden: { type: "boolean" },
+              purged: {
+                type: "boolean",
+                description: "Every member has now deleted it: the chat is gone from the server.",
+              },
+            },
+          }),
+        },
+        ...fail([404]),
+      },
+    },
+  },
   "/api/chat/rooms/{id}/leave": {
     post: {
       tags: ["Chat"],
@@ -262,6 +310,8 @@ const paths = {
     post: {
       tags: ["Chat"],
       summary: "Mark a room read up to a message",
+      description:
+        "Never moves back, never past the room's newest message. A mark that moved is told to the members as `chat_read` (`{roomId, userId, lastReadId}`).",
       parameters: [idParam],
       requestBody: {
         required: true,