Update `Cache-Control` with `stale-while-revalidate`/`stale-if-error` directives.

[midnight-wonderer]

I noticed that the current Cache-Control header is: public, max-age=31536000, immutable.
I wonder if max-age=31536000, public, stale-if-error=31536000, stale-while-revalidate=31536000, immutable would be better.

The additions are stale-if-error=31536000 and stale-while-revalidate=31536000.

• stale-while-revalidate will make Cloudflare revalidate asynchronously in the background, if the resource needs revalidation.
• stale-if-error will serve cached content when a request to the origin fails.

Note: The header must be accompanied by the appropriate CDN configuration for the desired effect.
For Cloudflare, this is in Cache Rules.
Make sure that Do not serve stale content while updating is set to off.