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/service/visit_ref_test.go

@@ -0,0 +1,31 @@
+package service
+
+import "testing"
+
+func TestClassifyReferrer(t *testing.T) {
+	cases := []struct {
+		raw, siteHost string
+		wantHost      string
+		wantKind      string
+	}{
+		{"", "example.com", "", RefKindDirect},
+		{"not a url", "example.com", "", RefKindDirect},
+		{"https://example.com/posts/1", "example.com", "example.com", RefKindInternal},
+		{"https://www.google.com/search?q=x", "example.com", "www.google.com", RefKindSearch},
+		{"https://www.baidu.com/s?wd=x", "example.com", "www.baidu.com", RefKindSearch},
+		{"https://cn.bing.com/search?q=x", "example.com", "cn.bing.com", RefKindSearch},
+		{"https://weibo.com/abc", "example.com", "weibo.com", RefKindSocial},
+		{"https://x.com/user/status/1", "example.com", "x.com", RefKindSocial},
+		{"https://v2ex.com/t/123", "example.com", "v2ex.com", RefKindSocial},
+		{"https://blog.other.org/post", "example.com", "blog.other.org", RefKindExternal},
+		{"https://google.com:443/search?q=x", "example.com", "google.com", RefKindSearch},
+		{"https://evil.com/?u=https://google.com", "example.com", "evil.com", RefKindExternal},
+	}
+	for _, c := range cases {
+		host, kind := ClassifyReferrer(c.raw, c.siteHost)
+		if host != c.wantHost || kind != c.wantKind {
+			t.Errorf("ClassifyReferrer(%q, %q) = (%q, %q), want (%q, %q)",
+				c.raw, c.siteHost, host, kind, c.wantHost, c.wantKind)
+		}
+	}
+}