afws: enable file logging with afws group permissions

nixbld-etc-nixos/afws-module.nix

@@ -10,16 +10,34 @@ in
       default = false;
       description = "Enable AFWS server";
     };
+    logFile = mkOption {
+      type = types.str;
+      default = "/var/lib/afws/logs/afws.log";
+      description = "Path to the log file";
+    };
+    logBackupCount = mkOption {
+      type = types.int;
+      default = 30;
+      description = "Number of daily log files to keep";
+    };
   };
 
   config = mkIf config.services.afws.enable {
     systemd.services.afws = {
       description = "AFWS server";
       wantedBy = [ "multi-user.target" ];
+      preStart = ''
+        mkdir -p "$(dirname ${config.services.afws.logFile})"
+        chown afws:afws "$(dirname ${config.services.afws.logFile})"
+      '';
       serviceConfig = {
         User = "afws";
         Group = "afws";
-        ExecStart = "${afws}/bin/afws_server";
+        ExecStart = ''
+          ${afws}/bin/afws_server \
+            --log-file ${config.services.afws.logFile} \
+            --log-backup-count ${toString config.services.afws.logBackupCount}
+        '';
         ExecReload = "${pkgs.coreutils}/bin/kill -USR1 $MAINPID";
       };
       path = [ pkgs.nix pkgs.git ];