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

backend/app/api/v1/practice_room.py

@@ -48,6 +48,13 @@ class JoinRoomRequest(BaseModel):
 class SendMessageRequest(BaseModel):
     """发送消息请求"""
     content: str = Field(..., description="消息内容")
+    source: Optional[str] = Field("text", description="消息来源: text/voice")
+
+
+class WebRTCSignalRequest(BaseModel):
+    """WebRTC 信令请求"""
+    signal_type: str = Field(..., description="信令类型: voice_offer/voice_answer/ice_candidate/voice_start/voice_end")
+    payload: dict = Field(..., description="信令数据（SDP/ICE候选等）")
 
 
 class RoomResponse(BaseModel):
@@ -399,6 +406,60 @@ async def send_message(
     }
 
 
+@router.post("/{room_code}/signal", summary="发送WebRTC信令")
+async def send_signal(
+    room_code: str,
+    request: WebRTCSignalRequest,
+    db: AsyncSession = Depends(get_db),
+    current_user: User = Depends(get_current_user)
+):
+    """
+    发送 WebRTC 信令消息
+    
+    信令类型：
+    - voice_start: 发起语音通话
+    - voice_offer: SDP Offer
+    - voice_answer: SDP Answer
+    - ice_candidate: ICE 候选
+    - voice_end: 结束语音通话
+    """
+    service = PracticeRoomService(db)
+    
+    # 获取房间
+    room = await service.get_room_by_code(room_code.upper())
+    if not room:
+        raise HTTPException(status_code=404, detail="房间不存在")
+    
+    # 检查用户是否在房间中
+    user_role = room.get_user_role(current_user.id)
+    if not user_role:
+        raise HTTPException(status_code=403, detail="您不是房间参与者")
+    
+    # 验证信令类型
+    valid_signal_types = ["voice_start", "voice_offer", "voice_answer", "ice_candidate", "voice_end"]
+    if request.signal_type not in valid_signal_types:
+        raise HTTPException(status_code=400, detail=f"无效的信令类型，必须是: {', '.join(valid_signal_types)}")
+    
+    # 发送信令消息（作为系统消息存储，用于 SSE 推送）
+    message = await service.send_message(
+        room_id=room.id,
+        user_id=current_user.id,
+        content=None,  # 信令消息不需要文本内容
+        role_name=None,
+        message_type=request.signal_type,
+        extra_data=request.payload
+    )
+    
+    return {
+        "code": 200,
+        "message": "信令发送成功",
+        "data": {
+            "signal_type": request.signal_type,
+            "sequence": message.sequence
+        }
+    }
+
+
 @router.get("/{room_code}/messages", summary="获取消息列表")
 async def get_messages(
     room_code: str,