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/store.go

@@ -48,6 +48,12 @@ CREATE INDEX IF NOT EXISTS checks_monitor ON checks(monitor_id, time);
 -- One row per monitor per UTC day, kept far longer than raw checks: it feeds the status page's uptime bars.
 CREATE TABLE IF NOT EXISTS daily (
 	monitor_id TEXT NOT NULL, day TEXT NOT NULL, checks INTEGER NOT NULL, fails INTEGER NOT NULL, PRIMARY KEY (monitor_id, day));
+-- Website analytics: one row per page view or event. No IP address or user agent is kept, only labels.
+CREATE TABLE IF NOT EXISTS hits (
+	website TEXT NOT NULL, time INTEGER NOT NULL, visitor TEXT NOT NULL, event TEXT NOT NULL, path TEXT NOT NULL,
+	referrer TEXT NOT NULL, browser TEXT NOT NULL, os TEXT NOT NULL, device TEXT NOT NULL, country TEXT NOT NULL,
+	lang TEXT NOT NULL, data TEXT NOT NULL);
+CREATE INDEX IF NOT EXISTS hits_site ON hits(website, time);
 CREATE TABLE IF NOT EXISTS mstate (
 	monitor_id TEXT PRIMARY KEY, status TEXT NOT NULL, since INTEGER NOT NULL, last_check INTEGER NOT NULL,
 	last_ms INTEGER NOT NULL, fails INTEGER NOT NULL, last_ping INTEGER NOT NULL, detail TEXT NOT NULL);
@@ -246,7 +252,8 @@ func (s *Store) events(scope, ref string) []Event {
 }
 
 // prune deletes data past its retention.
-func (s *Store) prune(now time.Time, messageDays, checkDays int) {
+func (s *Store) prune(now time.Time, messageDays, checkDays, hitDays int) {
+	s.db.Exec(`DELETE FROM hits WHERE time < ?`, now.AddDate(0, 0, -hitDays).Unix())
 	s.db.Exec(`DELETE FROM messages WHERE time < ?`, now.AddDate(0, 0, -messageDays).Unix())
 	s.db.Exec(`DELETE FROM checks WHERE time < ?`, now.AddDate(0, 0, -checkDays).Unix())
 	s.db.Exec(`DELETE FROM daily WHERE day < ?`, now.UTC().AddDate(0, 0, -400).Format("2006-01-02"))