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

internal/i18n/dict_en.go

@@ -951,4 +951,81 @@ var enDict = map[string]string{
 	"theme.name.sakura":           "Sakura",
 	"theme.name.shiroko":          "Shiroko",
 	"theme.name.new-sakura":       "New Sakura",
+
+	// Site statistics page (/stats): aggregate numbers only
+	"site.footer.stats": "Stats",
+	"stats.title":       "Site statistics",
+	"stats.lead":        "Only aggregate numbers here: how many questions the site receives, how fast they get answered, and when people tend to show up. No individual question, asker identity, or source address is shown.",
+	"stats.unavailable": "Statistics are temporarily unavailable. Please check back later.",
+
+	"stats.today.questions": "Asked today",
+	"stats.today.answers":   "Answered today",
+	"stats.today.tag":       "(live)",
+
+	"stats.kpi.users":       "Registered users",
+	"stats.kpi.questions":   "Questions asked",
+	"stats.kpi.answered":    "Answered",
+	"stats.kpi.answerRate":  "Answer rate",
+	"stats.kpi.activeBoxes": "Boxes that received questions",
+	"stats.kpi.messages":    "Direct messages",
+
+	"stats.insight.title":     "At a glance",
+	"stats.insight.note":      "Each line below is read straight from the numbers above — no guesswork.",
+	"stats.insight.empty":     "Not enough question data yet.",
+	"stats.insight.daily30":   "%s questions in the last 30 days, about %.1f per day.",
+	"stats.insight.peak":      "Busiest day was %s with %s questions.",
+	"stats.insight.busiest":   "Questions cluster around %s %d:00.",
+	"stats.insight.mom":       "Last month versus the month before: %s%% questions.",
+	"stats.insight.fastReply": "%s%% of answered questions got a reply within 24 hours.",
+	"stats.insight.private":   "%s%% of questions were sent privately.",
+
+	"stats.trend.dailyTitle":   "Questions over the last 30 days",
+	"stats.trend.dailyNote":    "Daily question count.",
+	"stats.trend.monthlyTitle": "Questions per month (last 12)",
+	"stats.trend.monthlyNote":  "The last bar is the current month, still in progress.",
+
+	"stats.mix.title":    "Composition",
+	"stats.mix.note":     "Three ratios: public or private, answered or not, and whether the asker was signed in.",
+	"stats.mix.public":   "Public questions",
+	"stats.mix.answered": "Answered",
+	"stats.mix.login":    "From signed-in users",
+
+	"stats.heat.title": "When questions arrive",
+	"stats.heat.note":  "Questions from the last %d days grouped by weekday and hour — darker means busier.",
+	"stats.heat.less":  "Less",
+	"stats.heat.more":  "More",
+
+	"stats.content.title":       "Length and pace",
+	"stats.content.note":        "How long questions are, and how fast they get answered.",
+	"stats.content.lengthTitle": "Question length",
+	"stats.content.replyTitle":  "Time to answer",
+
+	"stats.len.1": "under 20 chars",
+	"stats.len.2": "20–49 chars",
+	"stats.len.3": "50–99 chars",
+	"stats.len.4": "100–199 chars",
+	"stats.len.5": "200–499 chars",
+	"stats.len.6": "500+ chars",
+
+	"stats.reply.1": "within 1 hour",
+	"stats.reply.2": "1–6 hours",
+	"stats.reply.3": "6–24 hours",
+	"stats.reply.4": "1–3 days",
+	"stats.reply.5": "3–7 days",
+	"stats.reply.6": "over 7 days",
+
+	"stats.wd.1": "Mon",
+	"stats.wd.2": "Tue",
+	"stats.wd.3": "Wed",
+	"stats.wd.4": "Thu",
+	"stats.wd.5": "Fri",
+	"stats.wd.6": "Sat",
+	"stats.wd.7": "Sun",
+
+	"stats.foot.dataDate":   "Data as of",
+	"stats.foot.updated":    "computed at",
+	"stats.foot.prevMonth":  "Last question:",
+	"stats.foot.prevAnswer": "Last answer:",
+	"stats.foot.privacy":    "All figures are aggregates; no individual question, asker, or source address is shown.",
+	"stats.foot.cacheNote":  "Totals and charts are a snapshot taken at 00:00 local time, refreshed once a day; the two \"today\" figures are live.",
 }