## Summary
- cache successful public list responses for `/v0/servers` and
`/v0.1/servers` at ingress-nginx for 30 seconds
- give only those exact cached paths a 600 RPM per-controller budget
- preserve the existing 180 RPM limit for publish, auth, mutation,
detail, and other routes
- expose `X-Registry-Cache` to make cache MISS/HIT behavior directly
observable
## Why
Production ingress logs show that the current 180 RPM per-controller
limit rejects legitimate bulk pagination before it reaches the
application. The affected traffic is overwhelmingly registry-list reads,
not publishing.
A ten-minute production sample contained 17,836 successful list
responses but only 3,188 unique URLs: 82.1% were repeated requests,
representing roughly 1 GB of duplicate response bodies. Raising the list
limit without caching would increase pressure on the application and its
shared PostgreSQL pool. The short ingress cache absorbs repeated
cursor-page reads before raising their scoped client budget.
Publish latency is a separate concern. Reducing read pressure may help
indirectly, but slow external package validation is intentionally out of
scope for this PR.
Relates to #826.
## Safety
- caching is limited to exact public GET list paths and keyed by host
plus full request URI
- only 200 responses are cached
- cache lifetime is 30 seconds; newly published entries can therefore
take up to 30 seconds to appear on a cached page
- cache locking coalesces simultaneous misses
- stale cached responses may be served while refreshing or during
transient upstream 5xx/timeouts
- each ingress replica uses at most 1 GB of ephemeral on-disk cache
## Validation
- `go test ./...` in `deploy/`
- `go vet ./...` in `deploy/`
- `git diff --check`
- ingress-nginx chart 4.13.0 in a disposable kind cluster
- generated NGINX configuration passed `nginx -t`
- repeated list request verified `X-Registry-Cache: MISS` followed by
`HIT`
- non-list route verified uncached
- exact list locations verified with the higher rate zone while the
catch-all retained 180 RPM
Nothing was applied to production during investigation or validation.
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