feat: 站点访问统计与可配置侧边栏组件

- 新增访问统计：visitstats 服务与查询、visit_capture 中间件、爬虫/来源识别、管理端 analytics 页面
- 新增侧边栏：sidebar 服务与 handler、管理端 sidebar 配置页、前端侧边栏组件
- 新增 widget 嵌入代码生成（widgetCode）与 widgetApi
- 遥测逻辑迁移至 handler 层，删除 service/telemetry
- env 示例补充 Docker 部署可信代理 IP 说明

backend/model/db.go

@@ -58,10 +58,10 @@ func InitDB(dsn string) error {
 	if err := db.AutoMigrate(
 		&TemporaryUpload{}, &ModuleConfig{}, &SettingsAudit{}, &ActionCounter{}, &MailTask{}, &EmailChallenge{}, &StoredObject{},
 		&User{}, &Board{}, &Post{}, &Comment{}, &CommentEditHistory{}, &RefreshToken{}, &Like{}, &Notification{}, &Checkin{},
-		&Announcement{}, &SitePage{}, &SiteSetting{}, &SiteDailyStats{}, &SiteDailyVisitor{}, &Attachment{}, &UserBoard{}, &LoginLog{},
+		&Announcement{}, &SitePage{}, &SiteSetting{}, &SiteDailyStats{}, &SiteDailyVisitor{}, &VisitEvent{}, &Attachment{}, &UserBoard{}, &LoginLog{},
 		&ChatRoom{}, &ChatRoomMember{}, &ChatMessage{},
 		&PointLedger{}, &PostContentUnlock{}, &PostAttachment{}, &PostAttachmentUnlock{},
-		&PostPollVote{}, &PostLotteryEntry{}, &Ad{},
+		&PostPollVote{}, &PostLotteryEntry{}, &Ad{}, &SidebarWidget{},
 		&ImportRecord{},
 	); err != nil {
 		return fmt.Errorf("自动迁移失败: %w", err)