Add logger *slog.Logger to Middleware struct and thread it through New(). Replace all log.Println/log.Printf calls with mw.logger.Error/Warn using structured key-value pairs. Standalone functions (AuthMiddleware, Paginate) use slog.Default() to avoid signature breakage. Update router.go call site to pass s.logger. Signed-off-by: Matías Insaurralde <matias@insaurral.de>