feat: 添加双人对练语音通话功能

- 后端：扩展 SSE 支持 WebRTC 信令消息转发
- 前端：创建 WebRTC 连接管理模块 (webrtc.ts)
- 前端：创建 useVoiceCall 组合式函数
- 前端：在对练房间添加语音通话 UI
- 集成 Web Speech API 实现语音转文字

frontend/src/api/duoPractice.ts

@@ -76,17 +76,35 @@ export interface RoomDetailResponse {
   is_host: boolean
 }
 
+export type MessageType = 
+  | 'chat' 
+  | 'system' 
+  | 'join' 
+  | 'leave' 
+  | 'start' 
+  | 'end'
+  | 'voice_start'
+  | 'voice_offer'
+  | 'voice_answer'
+  | 'ice_candidate'
+  | 'voice_end'
+
 export interface RoomMessage {
   id: number
   room_id: number
   user_id?: number
-  message_type: 'chat' | 'system' | 'join' | 'leave' | 'start' | 'end'
+  message_type: MessageType
   content?: string
   role_name?: string
   sequence: number
   created_at: string
 }
 
+export interface WebRTCSignalRequest {
+  signal_type: 'voice_start' | 'voice_offer' | 'voice_answer' | 'ice_candidate' | 'voice_end'
+  payload: Record<string, any>
+}
+
 export interface MessagesResponse {
   messages: RoomMessage[]
   room_status: string
@@ -185,3 +203,20 @@ export function generateShareLink(roomCode: string): string {
   const baseUrl = window.location.origin
   return `${baseUrl}/trainee/duo-practice/join/${roomCode}`
 }
+
+/**
+ * 发送 WebRTC 信令
+ */
+export function sendSignal(roomCode: string, signalType: string, payload: Record<string, any>) {
+  return request.post(`/api/v1/practice/rooms/${roomCode}/signal`, {
+    signal_type: signalType,
+    payload
+  })
+}
+
+/**
+ * 获取对练报告
+ */
+export function getPracticeReport(roomCode: string) {
+  return request.get(`/api/v1/practice/rooms/${roomCode}/report`)
+}