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_test.go

@@ -0,0 +1,42 @@
+package storage
+
+import "testing"
+
+func TestMetrics_ViewsAndUniques(t *testing.T) {
+	setupTestDB(t)
+
+	// Three views on home, two on gaming.
+	RecordPageView("home")
+	RecordPageView("home")
+	RecordPageView("home")
+	RecordPageView("gaming")
+	RecordPageView("gaming")
+
+	// Two distinct visitors today; the repeat token must not double-count.
+	RecordVisitor("tok-a")
+	RecordVisitor("tok-a")
+	RecordVisitor("tok-b")
+
+	m := GetMetricsSummary()
+
+	if m.TotalViews != 5 {
+		t.Errorf("TotalViews = %d, want 5", m.TotalViews)
+	}
+	if m.ViewsToday != 5 {
+		t.Errorf("ViewsToday = %d, want 5", m.ViewsToday)
+	}
+	if m.UniquesToday != 2 {
+		t.Errorf("UniquesToday = %d, want 2 (deduped)", m.UniquesToday)
+	}
+	if len(m.Pages) == 0 || m.Pages[0].Path != "home" || m.Pages[0].Total != 3 {
+		t.Errorf("Pages[0] = %+v, want home with 3 views first", m.Pages)
+	}
+}
+
+func TestMetrics_EmptyIsZero(t *testing.T) {
+	setupTestDB(t)
+	m := GetMetricsSummary()
+	if m.TotalViews != 0 || m.ViewsToday != 0 || m.UniquesToday != 0 || len(m.Pages) != 0 {
+		t.Errorf("expected zero-value summary, got %+v", m)
+	}
+}