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/web/handlers.go

@@ -123,6 +123,7 @@ func (s *Server) base(r *http.Request) pageData {
 }
 
 func (s *Server) handleIndex(w http.ResponseWriter, r *http.Request) {
+	s.track(r, "home")
 	const (
 		justPostedLimit = 6
 		latestLimit     = 16
@@ -177,6 +178,7 @@ func (s *Server) handleIndex(w http.ResponseWriter, r *http.Request) {
 }
 
 func (s *Server) handleChannel(w http.ResponseWriter, r *http.Request, ch Channel) {
+	s.track(r, ch.Slug)
 	page := 1
 	if p := r.URL.Query().Get("page"); p != "" {
 		if n, err := strconv.Atoi(p); err == nil && n > 0 {