feat(features): add test factories to all 5 features

Adds src/__factories__/<entity>.factory.ts to auth, blog, marketing-pages,
navigation, media. Each factory uses defineFactory from @repo/core-testing
with stable date defaults (2026-01-01) so snapshot diffs reflect SUT
behavior only. Refactors mechanical inline-fixture tests to use factories.

Also adds vitest.config.ts and tsconfig path alias to @repo/media (lacked
both), and adds @repo/core-testing devDependency to @repo/media.

Spec: §5.1, §6.3

packages/navigation/src/__factories__/header.factory.ts

@@ -0,0 +1,6 @@
+import { defineFactory } from "@repo/core-testing/factory";
+import type { Header } from "../entities/header.js";
+
+export const headerFactory = defineFactory<Header>(() => ({
+  items: [],
+}));