Fix everything.

system/services/server/transmission/default.nix

@@ -0,0 +1,16 @@
+{ config, lib, pkgs, ... }:
+{
+  imports = [ ./nginx ];
+
+  config = lib.mkIf config.system.server.enable {
+    services.transmission = {
+      enable = true;
+      credentialsFile = pkgs.writeText "credentials" config.secrets.transmissionCredFile;
+      openPeerPorts = true;
+      settings.rpc-authentication-required = true;
+    };
+    environment.persistence."/persist".directories = [
+      "/var/lib/transmission"
+    ];
+  };
+}