only run 1 e2e test at a time

I think each test wipes the database at startup? but they share the
database, so running them in parallel feels like a bad idea in general

packages/backend/jest.config.e2e.cjs

@@ -12,4 +12,5 @@ module.exports = {
 	testMatch: [
 		"<rootDir>/test/e2e/**/*.ts",
 	],
+	maxConcurrency: 1, // these tests share a database, running more than one at a time is a bad idea
 };