#795 Deduplicate getPost calls in post page

Use React cache() to deduplicate getPost calls between
generateMetadata and page prefetch within the same request.

- Add getPostCached to cached-api.ts
- Update post layout and page to use cached version

This eliminates duplicate API calls for the same post data.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>