feat(access-logs): 接入 User-Agent 与设备/浏览器/状态码分布

- Agent 访问日志上报 user_agent，OpenResty log_format 输出 http_user_agent
- of_node_access_logs 新增 user_agent 列并写入 ClickHouse
- 访问日志概览新增：设备类型饼图、状态码饼图、浏览器/OS/User-Agent 排行
- 日志明细列表增加 User-Agent 列
- 扩展 UA 解析工具（browser/os/device）并支持 CLI 识别

internal/apps/openflare/agent/observability.go

@@ -24,6 +24,7 @@ const (
 	healthSeverityWarning       = "warning"
 	healthSeverityCritical      = "critical"
 	accessLogPathMaxLength      = 100
+	accessLogUserAgentMaxLength = 512
 	healthEventMessageMaxLength = 4096
 )
 
@@ -208,6 +209,7 @@ func buildNodeAccessLogRecords(nodeID string, direct []NodeAccessLog, buffered [
 				Region:        "",
 				Host:          strings.TrimSpace(item.Host),
 				Path:          truncateForDatabase(strings.TrimSpace(item.Path), accessLogPathMaxLength),
+				UserAgent:     truncateForDatabase(strings.TrimSpace(item.UserAgent), accessLogUserAgentMaxLength),
 				StatusCode:    item.StatusCode,
 				BytesSent:     bytesSent,
 				RequestLength: requestLength,