perf: rethink authenticated feed detail caching with role-based cache and client SWR

[aminamos]

Summary:

Resolves #48.
Replaces wasteful per-user server data cache (feed-complete-${userId}) with role-based caching (admin vs authenticated), adds client-side SWR caching hook (useFeedDetailCache), and documents architecture in docs/architecture/authenticated-feed-detail-caching.md.

Expected behavior:

Next.js server cache is no longer bloated by duplicate per-user feed data, while client retains instant SWR navigation.

Testing tips:

1. Checkout perf/48-authed-feed-detail-caching.
2. Run yarn test.

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• Add or update any needed documentation to the repo
• Format the title like "feat: [new feature short description]". Title must follow the Conventional Commit Specification(https://www.conventionalcommits.org/en/v1.0.0/).
• Linked all relevant issues
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