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/blog/src/__factories__/article.factory.ts

@@ -0,0 +1,13 @@
+import { defineFactory } from "@repo/core-testing/factory";
+import type { Article } from "../entities/article.js";
+
+export const articleFactory = defineFactory<Article>(({ sequence }) => ({
+  id: `article-${sequence}`,
+  title: `Article ${sequence}`,
+  slug: `article-${sequence}`,
+  content: null,
+  status: "draft",
+  authorId: "user-1",
+  createdAt: new Date("2026-01-01T00:00:00Z"),
+  updatedAt: new Date("2026-01-01T00:00:00Z"),
+}));