chore: set system user's email to systemuser@ in db

This matches the email that is used by the system user defined in
`core.ts`.

This is the companion PR to
#5845. With both of those
merged, the system user in the DB should match the one defined in `core.ts`

src/migrations/20240111075911-update-system-user-email.js

@@ -0,0 +1,15 @@
+'use strict';
+
+exports.up = function (db, callback) {
+    db.runSql(
+        `UPDATE users SET email = '[email protected]' WHERE id = -1337;`,
+        callback,
+    );
+};
+
+exports.down = function (db, callback) {
+    db.runSql(
+        `UPDATE users SET email = '[email protected]' WHERE id = -1337;`,
+        callback,
+    );
+};