fastly: Cache crates.io webapp responses per origin headers

[Turbo87]

The webapp Fastly service ran return(pass) in vcl_recv for every request, so it never cached anything. The comment said this was to avoid caching the API, but the same origin (crates-io.herokuapp.com) also serves the SvelteKit frontend, including immutable static assets that set Cache-Control: public, max-age=315360000. Those assets were never cached on the Fastly path, while the CloudFront distribution for the same origin cached them normally.

This removes the blanket pass so Fastly honors the origin's Cache-Control, matching CloudFront. default_ttl = 0 stays, so responses without caching headers and per-user no-store responses are still not cached.

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Created with the help of Claude Code, which:

• traced the missing cache hits to the unconditional return(pass) by inspecting Fastly vs CloudFront headers,
• reviewed the git history to understand why the snippet was added, and
• made the VCL change.

[marcoieni]

applied in staging, let me know how testing goes

[Turbo87]

x-served-by: cache-fra-eddf8230101-FRA, cache-fra-etou8220020-FRA
x-cache: MISS, HIT
x-cache-hits: 0, 3

this is what I'm now seeing for our static assets, while the regular API responses are still MISS, MISS, as intended 👍

[marcoieni]

should I deploy to prod?

[Turbo87]

yes, please

[marcoieni]

done!