services.nix: delete old generations, automatically run gc

modules/services.nix

@@ -151,5 +151,23 @@
         Persistent = true;
       };
     };
+    services."nix-prune-generations" = {
+      description = "Keep only 5 system generations and run nix-collect-garbage";
+      serviceConfig = {
+        Type = "oneshot";
+        ExecStart = ''
+          ${pkgs.nix}/bin/nix-env -p /nix/var/nix/profiles/system --delete-generations +5 || true
+          ${pkgs.nix}/bin/nix-collect-garbage -d || true
+        '';
+        Nice = "10";
+      };
+    };
+    timers."nix-prune-generations" = {
+      wantedBy = [ "timers.target" ];
+      timerConfig = {
+        OnCalendar = "weekly";
+        Persistent = true;
+      };
+    };
   };
 }