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/middleware/visit_capture.go

@@ -0,0 +1,62 @@
+package middleware
+
+import (
+	"strings"
+
+	"github.com/freefire/jiang13-bbs/model"
+	"github.com/freefire/jiang13-bbs/service"
+	"github.com/gin-gonic/gin"
+)
+
+// VisitCapture 服务端访问捕获：只记「UA 命中爬虫库」与「响应状态 ≥ 400」两类请求，
+// 普通人类浏览由客户端埋点上报（互补、不重复）。事件非阻塞入队，不影响响应。
+func VisitCapture(svc *service.VisitStatsService) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		c.Next()
+
+		if svc == nil || !svc.Enabled() {
+			return
+		}
+		// 静态资源与健康检查/WS 无统计意义
+		path := c.Request.URL.Path
+		if strings.HasPrefix(path, "/uploads") || path == "/health" || path == "/api/ws" {
+			return
+		}
+
+		ua := c.Request.UserAgent()
+		bot := service.ClassifyBot(ua)
+		status := c.Writer.Status()
+		if bot == "" && status < 400 {
+			return // 正常人类请求：不记
+		}
+
+		ev := model.VisitEvent{}
+		if bot != "" {
+			ev.Kind = model.VisitKindBot
+			ev.Bot = bot
+		} else {
+			ev.Kind = model.VisitKindProbe
+			ev.Status = status
+		}
+		if q := c.Request.URL.RawQuery; q != "" {
+			ev.Path = path + "?" + q
+		} else {
+			ev.Path = path
+		}
+		if len(ev.Path) > 512 {
+			ev.Path = ev.Path[:512]
+		}
+		ev.IP = service.CanonicalIP(c.ClientIP())
+		if vid, err := c.Cookie(service.VisitorCookieName); err == nil && service.ValidVisitorID(vid) {
+			ev.VidHash = service.HashVisitorID(vid)
+		}
+		if u := CurrentUser(c); u != nil {
+			ev.UserID = u.ID
+		}
+		p := service.ParseUserAgent(ua)
+		ev.Device = p.Kind
+		ev.OS = p.OS
+		ev.Browser = p.Browser
+		svc.Enqueue(ev)
+	}
+}