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/server.go

@@ -9,6 +9,7 @@ import (
 	"io/fs"
 	"log/slog"
 	"net/http"
+	"sync"
 	"time"
 
 	"pete/internal/config"
@@ -56,6 +57,12 @@ type Server struct {
 	srv     *http.Server
 	tpls    map[string]*template.Template // keyed by page name (e.g. "index", "channel")
 	auth    *Authenticator                // nil when sign-in is disabled or unavailable
+
+	// Daily-rotated salt for the privacy-preserving unique-visitor estimate.
+	// Guarded by metricsMu; never persisted (see metrics.go).
+	metricsMu sync.Mutex
+	saltDay   int64
+	salt      [16]byte
 }
 
 // New builds the server. Templates are parsed once at startup. Each page