test(helpers): add db, fetch, app, and auth test helpers

withTestDb sets up an in-memory SQLite handle with migrations
applied, swaps the production singleton via _setDbForTest, and
restores in a finally block. installFetchStub captures every
outbound request and throws if restored twice. requestApp wraps
Oak's app.handle so route tests can run without listen().
stubAuthenticate flips the server's authenticator indirection.

tests/_helpers/auth.ts

@@ -0,0 +1,18 @@
+import {
+  _resetAuthenticatorForTest,
+  _setAuthenticatorForTest,
+} from "../../server.ts";
+
+export type AuthStub = { restore: () => void };
+
+export const stubAuthenticate = (didOrError: string | Error): AuthStub => {
+  _setAuthenticatorForTest(() => {
+    if (didOrError instanceof Error) {
+      return Promise.reject(didOrError);
+    }
+    return Promise.resolve(didOrError);
+  });
+  return {
+    restore: () => _resetAuthenticatorForTest(),
+  };
+};