feat(api): collab 房间管理与 Yjs 帧转发

accept 时按库里的 adminType 复核身份，房间以学生为键。
服务端只按房间转发二进制帧，不解析内容。
老师掉线请求退回排队，学生掉线请求随人清除。

顺带处理三项 Task 2 评审遗留：
- TEACHER_ROLES 去重，handler.ts 改为从 routes/helpers.ts 导入，不再自留一份
- 补全学生排队中断线（尚未进房间）的清理分支，避免陈旧请求卡死在列表里
- 修正 websocket.ts 里一处过期注释：username/adminType 是三种 kind 都会填，
  不是只有 collab 才填

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01K1d8B3f4SXJwDvUY625eQd

apps/api/src/collab/handler.ts

@@ -2,22 +2,25 @@ import { and, eq, isNull } from "drizzle-orm"
 
 import { touchSession } from "../auth/session"
 import { db, schema } from "../db"
+import { TEACHER_ROLES } from "../routes/helpers"
 import {
   addRequest,
   addTeacher,
+  closeRoom,
   getRequest,
   hasTeacherOnline,
   listRequests,
+  openRoom,
   queueAheadOf,
   removeRequest,
   removeTeacher,
+  roomOf,
   teacherSockets,
   type CollabSocket,
   type HelpRequest,
+  type Room,
 } from "./state"
 
-const TEACHER_ROLES = ["Teacher Admin", "Super Admin"]
-
 function isTeacher(ws: CollabSocket) {
   return TEACHER_ROLES.includes(ws.data.adminType ?? "")
 }
@@ -64,7 +67,36 @@ export function handleCollabOpen(ws: CollabSocket) {
 
 export function handleCollabClose(ws: CollabSocket) {
   if (isTeacher(ws)) removeTeacher(ws)
-  // 房间与请求的清理在 Task 3 补
+
+  const room = roomOf(ws)
+  if (room) {
+    closeRoom(room.studentId)
+    room.studentSocket.data.roomOwnerId = undefined
+    room.teacherSocket.data.roomOwnerId = undefined
+    const peer = ws === room.teacherSocket ? room.studentSocket : room.teacherSocket
+    peer.send(JSON.stringify({ type: "room_closed", reason: "peer_offline" }))
+
+    if (ws === room.teacherSocket) {
+      // 老师掉线：请求退回排队，学生不必重新点 —— 可能只是网络抖了一下
+      const request = getRequest(room.studentId)
+      if (request) {
+        request.status = "pending"
+        request.teacherId = undefined
+        request.teacherName = undefined
+        sendHelpStatus(request.socket, "pending", {
+          queueAhead: queueAheadOf(room.studentId),
+        })
+      }
+    } else {
+      // 学生掉线：请求随人走
+      removeRequest(room.studentId)
+    }
+  } else if (!isTeacher(ws)) {
+    // 还在排队时关掉页面，请求也该消失
+    removeRequest(ws.data.userId)
+  }
+
+  broadcastRequests()
 }
 
 export async function handleCollabMessage(ws: CollabSocket, raw: string) {
@@ -95,6 +127,15 @@ export async function handleCollabMessage(ws: CollabSocket, raw: string) {
     case "help_cancel":
       handleHelpCancel(ws)
       return
+    case "accept":
+      await handleAccept(ws, message.studentId)
+      return
+    case "reject":
+      handleReject(ws, message.studentId)
+      return
+    case "leave":
+      handleLeave(ws)
+      return
     default:
       ws.send(JSON.stringify({ type: "error", message: "Invalid message" }))
   }
@@ -161,5 +202,111 @@ function handleHelpCancel(ws: CollabSocket) {
   broadcastRequests()
 }
 
-/** Task 3 会把它换成真正的按房间转发。此刻房间还不存在，先收下不处理 */
-export function handleCollabBinary(_ws: CollabSocket, _data: Buffer | Uint8Array) {}
+async function handleAccept(ws: CollabSocket, studentId: unknown) {
+  if (!isTeacher(ws)) {
+    ws.send(JSON.stringify({ type: "error", message: "无权限" }))
+    return
+  }
+  if (typeof studentId !== "number") {
+    ws.send(JSON.stringify({ type: "error", message: "Invalid studentId" }))
+    return
+  }
+
+  // 握手时的 adminType 是那一刻的快照，接单前按库里的真实身份复核一次。
+  // 注意读的是库，不是前端传的任何东西 —— 前端的演示模式在这里没有意义
+  const [teacher] = await db
+    .select({ adminType: schema.user.adminType })
+    .from(schema.user)
+    .where(and(eq(schema.user.id, ws.data.userId), eq(schema.user.isDisabled, false)))
+    .limit(1)
+  if (!teacher || !TEACHER_ROLES.includes(teacher.adminType)) {
+    ws.close(1008, "Permission revoked")
+    return
+  }
+
+  // 老师同时只能在一个房间
+  if (roomOf(ws)) {
+    ws.send(JSON.stringify({ type: "error", message: "请先退出当前协作" }))
+    return
+  }
+
+  const request = getRequest(studentId)
+  if (!request || request.status === "active") {
+    // 被别人接走了或者学生已经撤销 —— 回一份最新列表让老师端自己纠正
+    ws.send(
+      JSON.stringify({ type: "requests", list: listRequests().map(serializeRequest) }),
+    )
+    return
+  }
+
+  request.status = "active"
+  request.teacherId = ws.data.userId
+  request.teacherName = ws.data.username ?? ""
+
+  ws.data.roomOwnerId = studentId
+  request.socket.data.roomOwnerId = studentId
+  openRoom({
+    studentId,
+    teacherId: ws.data.userId,
+    studentSocket: request.socket,
+    teacherSocket: ws,
+    problemId: request.problemId,
+  })
+
+  const openFrame = (peerName: string, peerRole: "student" | "teacher") =>
+    JSON.stringify({
+      type: "room_open",
+      peer: { name: peerName, role: peerRole },
+      problemId: request.problemId,
+    })
+  request.socket.send(openFrame(request.teacherName, "teacher"))
+  ws.send(openFrame(request.studentName, "student"))
+  sendHelpStatus(request.socket, "active", { teacherName: request.teacherName })
+  broadcastRequests()
+}
+
+function handleReject(ws: CollabSocket, studentId: unknown) {
+  if (!isTeacher(ws) || typeof studentId !== "number") return
+  const request = getRequest(studentId)
+  // 已经在协作中的不能靠 reject 掐掉，那是 leave 的事
+  if (!request || request.status === "active") return
+  removeRequest(studentId)
+  sendHelpStatus(request.socket, "cancelled")
+  broadcastRequests()
+}
+
+/** 主动退出房间。老师点关闭、学生点结束都走这里 */
+function handleLeave(ws: CollabSocket) {
+  const room = roomOf(ws)
+  if (!room) return
+  teardownRoom(room, "done")
+}
+
+/**
+ * 拆房间。reason 决定两端看到什么：
+ *   done         —— 有人主动结束，双方都收到，请求一并清除
+ *   peer_offline —— 有人断线，见 handleCollabClose
+ */
+function teardownRoom(room: Room, reason: "done" | "peer_offline") {
+  closeRoom(room.studentId)
+  room.studentSocket.data.roomOwnerId = undefined
+  room.teacherSocket.data.roomOwnerId = undefined
+  const frame = JSON.stringify({ type: "room_closed", reason })
+  room.studentSocket.send(frame)
+  room.teacherSocket.send(frame)
+  if (reason === "done") removeRequest(room.studentId)
+  broadcastRequests()
+}
+
+/**
+ * Yjs 的 update / awareness 帧。服务端不解析、不留存，只转发给房间里的另一个人。
+ *
+ * 「服务端不知道代码内容」是有意的：这个通道要做的事只有认证和分房间，
+ * 权限由 accept 时的库查询决定，与帧里装的是什么无关。
+ */
+export function handleCollabBinary(ws: CollabSocket, data: Buffer | Uint8Array) {
+  const room = roomOf(ws)
+  if (!room) return
+  const peer = ws === room.teacherSocket ? room.studentSocket : room.teacherSocket
+  peer.send(data)
+}