feat(stats): 新增公开数据页 /stats，入口收敛到页脚

首页底部那张「站点统计」卡片下线，完整数据移到公开只读页 /stats：
今日实时数、概览六格、近 30 天折线与 12 个月柱状、三个占比环、
7×24 时段热力图、提问字数与回答耗时分布。

- 聚合在 internal/db/analytics.go（SUM(CASE WHEN)，方言差异收敛在 analyticsDialectFor）
- 结果按自然日缓存，跨日重算一次后全站共用；重算失败降级返回上一份快照
- 图表零前端依赖：服务端算坐标的 SVG + CSS grid，禁用 JS 页面仍完整
- 环比取「上月 vs 上上月」：本月未过完，否则每月 1 号会得出 -95%
- 入口只在页脚「社区规则」与「找站长」之间；route.Home 不再计算统计
- 补 questions(created_at) 索引；启动后异步预热当天快照
- 测试：analytics 连真库验证软删除不计入、分桶下标、DOW 对齐

templates/zz_preview_test.go

@@ -181,6 +181,96 @@ func TestGenPreview(t *testing.T) {
 	writeQuestionOwnerPreviews(t, outDir)
 	writeAdminListPreviews(t, outDir)
 	writeVotePreviews(t, outDir)
+	writeStatsPreviews(t, outDir)
+}
+
+// writeStatsPreviews 全站数据页（/stats）。
+// 四种组合都出：亮/暗确认令牌适配，中/英确认英文长句不会把卡片挤变形。
+func writeStatsPreviews(t *testing.T, outDir string) {
+	t.Helper()
+	now := time.Now()
+
+	daily := make([]stats.DailyPoint, 0, 30)
+	var peak stats.DailyPoint
+	for i := 0; i < 30; i++ {
+		d := now.AddDate(0, 0, -(29 - i))
+		n := int64(38 + (i*11)%47 + (i%5)*13)
+		p := stats.DailyPoint{Date: d.Format("2006-01-02"), Label: d.Format("01-02"), N: n}
+		daily = append(daily, p)
+		if p.N >= peak.N {
+			peak = p
+		}
+	}
+
+	monthly := make([]stats.MonthlyPoint, 0, 12)
+	for i := 0; i < 12; i++ {
+		m := now.AddDate(0, -(11 - i), 0)
+		monthly = append(monthly, stats.MonthlyPoint{
+			Month: m.Format("2006-01"), Label: m.Format("01"),
+			N: int64(2100 + i*310 + (i%3)*420), Current: i == 11,
+		})
+	}
+
+	// 热力图：造出「深夜 + 周末」两个高峰，便于肉眼确认档位分得开。
+	var heatmap [7][24]int64
+	for d := 0; d < 7; d++ {
+		for h := 0; h < 24; h++ {
+			v := int64(30 + (h%12)*16)
+			if h >= 21 || h <= 1 {
+				v += 320
+			}
+			if h >= 12 && h <= 14 {
+				v += 90
+			}
+			if d >= 5 {
+				v += 120
+			}
+			heatmap[d][h] = v
+		}
+	}
+
+	ov := &stats.Overview{
+		RegisteredUsers: 1284, TotalQuestions: 56321, Answered: 31208,
+		Public: 41002, Private: 15319, Read: 40122, Unread: 16199,
+		Anonymous: 22110, LoggedIn: 34211,
+		ActiveBoxes: 412, AnsweringBoxes: 268, TotalMessages: 8842,
+		LastQuestionAt: "2026-10-22 23:41:07", LastAnswerAt: "2026-10-22 23:12:33",
+		AnswerRate: 55.4, FastReplyRate: 38.6, Total30: 2148, AvgDailyLast30: 71.6,
+		GrowthMoM: 12.3, HasPrevMonth: true,
+		TodayQuestions: 37, TodayAnswers: 21,
+		Daily: daily, Monthly: monthly, PeakDay: peak, DailyMax: peak.N, MonthlyMax: 6180,
+		BusiestWD: 6, BusiestHr: 23,
+		Heatmap: heatmap, HeatmapMax: 812, HeatmapDays: 180,
+		LengthBins: [6]int64{8200, 14200, 16800, 9200, 6100, 1821},
+		ReplyBins:  [6]int64{4200, 7100, 8400, 6200, 3400, 1908},
+		DataDate:   now.Format("2006-01-02"),
+		UpdatedAt:  now.Format("2006-01-02") + " 00:07:12",
+	}
+
+	for _, lang := range []string{"zh-CN", "en"} {
+		data := map[string]interface{}{
+			"Title":          "全站数据",
+			"LangCode":       lang,
+			"SupportedLangs": []map[string]string{{"Code": "zh-CN", "Name": "简体中文"}, {"Code": "en", "Name": "English"}},
+			"IsLogged":       false,
+			"IsUserPage":     false,
+			"IsPixel":        false,
+			"VConsole":       false,
+			"CurrentURI":     "/stats",
+			"ExternalURL":    "https://box.shiroko.one",
+			"CaptchaType":    "math",
+			"Limits":         conf.Limits,
+			"ICP":            "",
+			"Overview":       stats.NewPage(lang, ov),
+		}
+		for _, theme := range []string{"light", "dark"} {
+			page, err := renderToString(t, "stats", data)
+			if err != nil {
+				t.Fatalf("render stats: %v", err)
+			}
+			writePreview(t, outDir, "preview-stats-"+lang+"-"+theme+".html", page, theme, "../static")
+		}
+	}
 }
 
 // writeQuestionItemPreviews 单题页（/_/{domain}/{qid}）。
@@ -383,6 +473,9 @@ func writePreview(t *testing.T, dir, name, html, theme, staticPrefix string) {
 		}
 	}
 	html = strings.ReplaceAll(html, `href="/favicon.ico"`, `href="`+staticPrefix+`/favicon.ico"`)
+	// base.css 也要改成本地相对路径：预览页走 file://，"/static/base.css" 是绝对路径取不到，
+	// 于是 :root 上的设计令牌全部缺失 —— var(--tb-*) 变成无效值，卡片没边框、图表全透明。
+	html = strings.ReplaceAll(html, `href="/static/`, `href="`+staticPrefix+`/`)
 	html = strings.Replace(html, `<title>`, `<title>预览 `+time.Now().Format("15:04")+` · `, 1)
 	if err := os.WriteFile(filepath.Join(dir, name), []byte(html), 0o644); err != nil {
 		t.Fatal(err)