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/db.go

@@ -116,6 +116,11 @@ func RunMaintenance() {
 	exec("prune old reactions",
 		`DELETE FROM reactions WHERE reacted_at < ?`, storyCutoff)
 
+	// Daily unique tokens are only useful for the recent window; their salts are
+	// long gone. page_views is kept forever (tiny aggregate, all-time totals).
+	exec("prune old daily_visitors",
+		`DELETE FROM daily_visitors WHERE day < ?`, unixDay()-30)
+
 	exec("wal checkpoint", "PRAGMA wal_checkpoint(TRUNCATE)")
 	exec("optimize", "PRAGMA optimize")
 }