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/vitest.config.ts

@@ -0,0 +1,9 @@
+import path from "node:path";
+import { mergeConfig } from "vitest/config";
+import { nodeVitestConfig } from "@repo/core-typescript/vitest.base.node";
+
+export default mergeConfig(nodeVitestConfig, {
+  resolve: {
+    alias: { "@": path.resolve(__dirname, "./src") },
+  },
+});