feat: 添加 HTTP 请求日志中间件（Phase 4）

实现内容：
- 创建 trace/gin_logger.go，实现 GinLogger 和 GinRecovery 中间件
- GinLogger 自动记录所有 HTTP 请求的 method/path/status/latency/client_ip
- GinRecovery 使用 zap 记录 panic 恢复信息，替代 gin.Recovery()
- 修改 main.go 注册三层中间件：TraceMiddleware -> GinLogger -> GinRecovery
- 所有日志自动附加 trace_id 和 request_id 字段

日志示例：
{
  "level": "info",
  "ts": "2026-06-21T22:26:07.610+0800",
  "msg": "request completed",
  "trace_id": "01KVN964KSY6BFDKB9S932NXB8",
  "request_id": "01KVN964KSY6BFDKB9S932NXB8",
  "method": "GET",
  "path": "/api/health",
  "status": 200,
  "latency_ms": 0,
  "client_ip": "::1"
}

测试：已验证健康检查接口日志正常输出

backend/cmd/server/main.go

@@ -23,6 +23,7 @@ import (
 	"github.com/hhs/camtalk/internal/ratelimit"
 	"github.com/hhs/camtalk/internal/session"
 	"github.com/hhs/camtalk/internal/store"
+	"github.com/hhs/camtalk/internal/trace"
 	"github.com/hhs/camtalk/internal/ws"
 	migrations "github.com/hhs/camtalk/migrations"
 )
@@ -226,7 +227,9 @@ func main() {
 	}
 
 	r := gin.New()
-	r.Use(gin.Recovery())
+	r.Use(trace.TraceMiddleware())    // 第一层：生成 trace ID
+	r.Use(trace.GinLogger())          // 第二层：记录请求
+	r.Use(trace.GinRecovery())        // 第三层：panic 恢复
 
 	// REST API
 	apiGroup := r.Group("/api")