Add server-side web usage metrics with !petestats command

Track per-page/per-channel view counts and a privacy-preserving daily
unique-visitor estimate (salted IP+UA hash, salt rotated daily and never
persisted). No third-party analytics, no JS beacon. Surfaced via the
admin-gated !petestats Matrix command (named to avoid an existing !stats
bot in the rooms).

internal/storage/metrics.go

@@ -0,0 +1,114 @@
+package storage
+
+import (
+	"log/slog"
+	"sort"
+)
+
+const secondsPerDay = 86400
+
+// unixDay returns the current UTC day number (floor(unix / 86400)).
+func unixDay() int64 { return nowUnix() / secondsPerDay }
+
+// RecordPageView increments the all-time and per-day view counter for a coarse
+// path label ("home", a channel slug, …). Fire-and-forget: a failure here must
+// never affect serving a page, so errors are logged and swallowed.
+func RecordPageView(path string) {
+	exec("record page view",
+		`INSERT INTO page_views (path, day, views) VALUES (?, ?, 1)
+		 ON CONFLICT(path, day) DO UPDATE SET views = views + 1`,
+		path, unixDay())
+}
+
+// RecordVisitor records a (already-hashed, salted) visitor token for today's
+// unique estimate. Duplicate tokens within the same day are ignored.
+func RecordVisitor(token string) {
+	exec("record visitor",
+		`INSERT OR IGNORE INTO daily_visitors (day, visitor) VALUES (?, ?)`,
+		unixDay(), token)
+}
+
+// PathStat is one row of the per-page usage breakdown.
+type PathStat struct {
+	Path  string
+	Total int // all-time views
+	Today int // views since the start of the current UTC day
+}
+
+// DayStat is one day's unique-visitor estimate.
+type DayStat struct {
+	Day     int64 // unix day number
+	Uniques int
+}
+
+// MetricsSummary is the aggregate usage readout surfaced by !petestats.
+type MetricsSummary struct {
+	Pages        []PathStat // per-path, busiest first
+	TotalViews   int        // all-time, all paths
+	ViewsToday   int        // all paths, current UTC day
+	UniquesToday int        // distinct visitor tokens, current UTC day
+	Last7Days    []DayStat  // daily uniques, oldest → newest (note: salt rotates
+	// daily, so these CANNOT be summed into a cross-day unique count)
+}
+
+// GetMetricsSummary assembles the usage readout. Best-effort: any sub-query
+// failure leaves that field at its zero value rather than failing the whole
+// command.
+func GetMetricsSummary() MetricsSummary {
+	today := unixDay()
+	var m MetricsSummary
+
+	rows, err := Get().Query(
+		`SELECT path,
+		        SUM(views)                                    AS total,
+		        COALESCE(SUM(CASE WHEN day = ? THEN views END), 0) AS today
+		 FROM page_views
+		 GROUP BY path`, today)
+	if err != nil {
+		slog.Error("metrics: page_views query failed", "err", err)
+	} else {
+		defer rows.Close()
+		for rows.Next() {
+			var p PathStat
+			if err := rows.Scan(&p.Path, &p.Total, &p.Today); err != nil {
+				slog.Error("metrics: scan page stat failed", "err", err)
+				continue
+			}
+			m.Pages = append(m.Pages, p)
+			m.TotalViews += p.Total
+			m.ViewsToday += p.Today
+		}
+	}
+	sort.Slice(m.Pages, func(i, j int) bool {
+		if m.Pages[i].Total != m.Pages[j].Total {
+			return m.Pages[i].Total > m.Pages[j].Total
+		}
+		return m.Pages[i].Path < m.Pages[j].Path
+	})
+
+	if err := Get().QueryRow(
+		`SELECT COUNT(*) FROM daily_visitors WHERE day = ?`, today,
+	).Scan(&m.UniquesToday); err != nil {
+		slog.Error("metrics: uniques-today query failed", "err", err)
+	}
+
+	since := today - 6 // inclusive 7-day window
+	vrows, err := Get().Query(
+		`SELECT day, COUNT(*) FROM daily_visitors
+		 WHERE day >= ? GROUP BY day ORDER BY day`, since)
+	if err != nil {
+		slog.Error("metrics: 7-day uniques query failed", "err", err)
+	} else {
+		defer vrows.Close()
+		for vrows.Next() {
+			var d DayStat
+			if err := vrows.Scan(&d.Day, &d.Uniques); err != nil {
+				slog.Error("metrics: scan day stat failed", "err", err)
+				continue
+			}
+			m.Last7Days = append(m.Last7Days, d)
+		}
+	}
+
+	return m
+}