Make sure that PDFs proxied by NGINX do get cached in Cloudflare

[seanh]

The NGINX /proxy/static/ URLs for proxying PDFs need to end in ".pdf" or Cloudflare won't cache them (regardless of caching headers). See: https://hypothes-is.slack.com/archives/C1MA4E9B9/p1630943273021700?thread_ts=1630925851.005100&cid=C1MA4E9B9

In practice we only use this endpoint for proxying PDFs so we can probably get away with it always ending in ".pdf"?

NGINX may also need to strip caching-related headers from the third-party response and add its own caching headers in order to get Cloudflare to cach the responses. It looks like NGINX already does this:

via/conf/nginx/nginx.conf

Lines 86 to 89 in c22a5a0

	# Cache for one day, but allow serving from cache while revalidating for a week.
	# https://web.dev/stale-while-revalidate/
	proxy_hide_header "Cache-Control";
	add_header "Cache-Control" "public, max-age=86400, stale-while-revalidate=604800";

See #31 and #76

[seanh]

We could also create a Cloudflare page rule for the endpoint, then we wouldn't need to add/remove caching headers and the URLs wouldn't need to end in ".pdf".

Slack thread: https://hypothes-is.slack.com/archives/C1MA4E9B9/p1632303338068500

[marcospri]

We could also create a Cloudflare page rule for the endpoint, then we wouldn't need to add/remove caching headers and the URLs wouldn't need to end in ".pdf".

Created rule today. Waiting a day or two to see results.