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
