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Demo Mode — Calls PATCH /api/posts/how-caching-works with the dev secret.

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# How On-Demand Revalidation Works

## The Flow

1. **First request** — middleware checks `caches.default` (miss), renders the page, stores it, returns it with `Cache-Control: public, s-maxage=31536000`.
2. **Subsequent requests** — middleware finds the cached entry (hit) and returns it instantly — no D1 query, no rendering.
3. **Publishing a post** — the `/api/posts` endpoint inserts into D1, then calls `caches.default.delete()` for `/` and `/blog`.
4. **Next request** — cache miss again, fresh render picks up the new post, re-cached.

## Why not Cloudflare CDN cache headers alone?

`Cache-Control` headers tell Cloudflare's CDN to cache, but you cannot purge individual URLs without a paid Cache Purge API plan. The **Cache API** (`caches.default`) is per-worker, gives you programmatic `put` / `delete` control, and is free.

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