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/media/src/__factories__/media.factory.ts

@@ -0,0 +1,19 @@
+import { defineFactory } from "@repo/core-testing/factory";
+
+export interface Media {
+  id: string;
+  alt: string;
+  url: string;
+  filename: string;
+  mimeType: string;
+  filesize: number;
+}
+
+export const mediaFactory = defineFactory<Media>(({ sequence }) => ({
+  id: `media-${sequence}`,
+  alt: `Media ${sequence}`,
+  url: `https://cdn.example.com/media-${sequence}.png`,
+  filename: `media-${sequence}.png`,
+  mimeType: "image/png",
+  filesize: 1024,
+}));