feat: migrate echo-http to envoy gateway

[TineoC]

Changes

• Migrate echo-http service to Gateway API (HTTPRoute)

Depends on: #131

[TineoC]

is this not needed anymore?

[themightychris]

Superseded by the Envoy Gateway migration that landed in PRs #150, #152, #154, #156, #157. Closing.

The end state for echo-http matches what this PR was reaching for, just with the broader infrastructure in place to support it:

• _gateways/echo-http.yaml — per-app Gateway (HTTPS listener on echo-http.sandbox.k8s.phl.io with cert-manager annotation) + HTTPRoute attached to the per-app Gateway only (HTTP→HTTPS handled globally by _infra/envoy-gateway/http-redirect.yaml)
• Old nginx-style Ingress removed from echo-http.yaml
• echo-http-gw-tls cert auto-issued via the gatewayHTTPRoute solver on letsencrypt-prod

The pattern is the same one we used for every app on sandbox — see PR #152 for the full set and the design rationale.