feat: 初始化 Go 后端项目骨架

- 使用 Gin 框架替代原始 net/http，方便调试
- 实现 WebSocket 端点 (/ws)，支持 ping/query/config/interrupt 消息
- 实现健康检查 REST 接口 (/api/health)
- 定义协议数据结构 (models)
- 心跳检测（60 秒超时断开）

backend/cmd/server/main.go

@@ -0,0 +1,40 @@
+package main
+
+import (
+	"log"
+	"time"
+
+	"github.com/gin-gonic/gin"
+
+	"github.com/hhs/camtalk/internal/ws"
+)
+
+var startTime = time.Now()
+
+func main() {
+	r := gin.Default()
+
+	// REST API
+	api := r.Group("/api")
+	{
+		api.GET("/health", healthHandler)
+	}
+
+	// WebSocket
+	r.GET("/ws", ws.ServeWS)
+
+	log.Println("CamTalk gateway starting on :8080")
+	if err := r.Run(":8080"); err != nil {
+		log.Fatalf("failed to start server: %v", err)
+	}
+}
+
+// healthHandler 健康检查。
+func healthHandler(c *gin.Context) {
+	c.JSON(200, gin.H{
+		"status":         "ok",
+		"version":        "0.1.0",
+		"uptime":         time.Since(startTime).String(),
+		"active_sessions": 0, // TODO: 接入 Session Manager
+	})
+}