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/tsconfig.json

@@ -2,7 +2,11 @@
   "extends": "@repo/core-typescript/base.json",
   "compilerOptions": {
     "outDir": "dist",
-    "lib": ["ES2022", "DOM"]
+    "rootDir": ".",
+    "lib": ["ES2022", "DOM"],
+    "paths": {
+      "@/*": ["./src/*"]
+    }
   },
   "include": ["src/**/*"],
   "exclude": ["node_modules", "dist"]