Gravar-mail: catena-Poly[[(4-methyl­benzoato-κO)manganese(II)]-μ-aqua-bis­(μ-4-methyl­benzoato-κ(2) O:O′)[(4-methyl­benzoato-κO)manganese(II)]-bis­(μ-N,N-diethyl­nicotinamide)-κ(2) N (3):O;O:N (3)]