Add website analytics

A Tianji-style Websites section: a small tracking script, a public collection endpoint, and a
page per website with visitors, page views, visits, bounce rate, a chart and top lists for pages,
referrers, browsers, systems, devices, countries, languages and events. No cookies and no stored
IP address or user agent. SANDBAR_TRUST_PROXY makes the client address come from the proxy.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/sandbar/bars_test.go

@@ -38,7 +38,7 @@ func TestUptimeBars(t *testing.T) {
 	}
 
 	// The rollup survives pruning the raw checks.
-	s.st.prune(clk.Now().AddDate(0, 0, 60), 30, 1)
+	s.st.prune(clk.Now().AddDate(0, 0, 60), 30, 1, 365)
 	if got := s.dailyUptime(m.ID, 5, clk.Now()); got[2] != [2]int{4, 1} {
 		t.Fatalf("rollup lost after pruning: %v", got)
 	}