ghp: serve plain HTTP behind the gateway (fix redirect loop) (#361)

https://ghp.unkin.net/ 308-loops onto itself. The traefik gateway terminates TLS and forwards cleartext to the ghp Service port 80 -> container :8080, but :8080 was `GHP_SERVER_HTTP_LISTEN` — ghp's http->https **308 redirect** listener. So ghp bounced every request back to https, the gateway re-forwarded it to :8080, and it looped forever.

## Root cause (confirmed against ghp source)
`internal/server/server.go` `Run()` picks the serving mode:
```go
hasTLS := s.cfg.Server.HTTPSListen != "" || (systemd socket + certs)
if hasTLS { return s.serveTLS(...) }  // app on HTTPSListen (TLS); HTTPListen = 308 redirect; Listen IGNORED
return s.servePlain(...)              // app (full handler: mgmt UI + API) on Listen, cleartext
```
It is **strictly either/or**. `serveTLS` serves the app on `HTTPSListen` and gives `HTTPListen` only `httpsRedirectHandler()` (`redirect.go`: `http.StatusPermanentRedirect` = 308). `servePlain` serves the same full handler on `Listen` in cleartext (`createListener()` uses `cfg.Server.Listen`). The mgmt UI is the same `handler` in both modes, so it IS served on the plain `Listen` port.

Behind a TLS-terminating gateway that forwards cleartext to :8080, ghp therefore has to run in **plain mode**. Keeping `GHP_SERVER_HTTPS_LISTEN` would keep `hasTLS` true, leave `GHP_SERVER_LISTEN` ignored, and nothing would serve cleartext on :8080.

## Change
- **configmap**: drop `GHP_SERVER_HTTPS_LISTEN` and `GHP_SERVER_HTTP_LISTEN`; set `GHP_SERVER_LISTEN: ":8080"` so :8080 SERVES the app; add `GHP_SERVER_TRUST_PROXY_HEADERS: "true"` so ghp trusts the gateway's `X-Forwarded-*`/`Forwarded` for scheme/host (`GHP_SERVER_BASE_URL` already set).
- **deployment + vmservicescrape**: the metrics server only wraps TLS when `hasTLS` is true (`Run()` gates `loadTLSConfig` on `hasTLS`); in plain mode it is cleartext, so the `/metrics` liveness/readiness probes and the VMServiceScrape switch from HTTPS/https to HTTP/http.

Service, HTTPRoute and Gateway are unchanged. configmap+deployment carry the stakater reloader annotation, so pods roll on the change.

## Deviation from the brief
The brief said to keep `GHP_SERVER_HTTPS_LISTEN: ":8443"`. Source shows that is incompatible with serving cleartext on :8080 (the two modes are mutually exclusive), so this drops it. The unused `GHP_TLS_CERT_FILE`/`KEY_FILE`, the `tls` volume, and containerPort 8443 are left in place (harmless) for an easy revert to TLS mode. The alternative — gateway -> Service 443 -> :8443 with a BackendTLSPolicy — is the bigger change flagged in the brief and is NOT taken here.

Validated: `kustomize build apps/overlays/au-syd1/ghp` clean, kubeconform 0 invalid/0 errors, pre-commit clean. Not applied.
---------

Co-authored-by: unkin-agent <agent@unkin.net>
Reviewed-on: #361
Co-authored-by: Unkin Agent <unkin-agent@unkin.net>
Co-committed-by: Unkin Agent <unkin-agent@unkin.net>

apps/base/ghp/configmap.yaml

@@ -6,9 +6,21 @@ metadata:
   namespace: ghp
 data:
   GHP_DATABASE_DRIVER: postgres
+  # ghp runs in plain-HTTP reverse-proxy mode: the traefik gateway terminates TLS
+  # for ghp.unkin.net and forwards cleartext to container :8080, where ghp SERVES
+  # the full app (mgmt UI + API) via GHP_SERVER_LISTEN.
+  #
+  # Do NOT set GHP_SERVER_HTTPS_LISTEN here. Any non-empty https_listen flips ghp
+  # into TLS-only mode (server.go Run(): hasTLS -> serveTLS): the app is served on
+  # :8443 and http_listen becomes a 308 http->https REDIRECT listener, while
+  # GHP_SERVER_LISTEN is ignored entirely. The gateway forwarding cleartext into
+  # that redirect listener on :8080 was the ghp.unkin.net -> ghp.unkin.net 308 loop.
+  #
   # nonroot cannot bind <1024; listen high and remap in the Service.
-  GHP_SERVER_HTTPS_LISTEN: ":8443"
-  GHP_SERVER_HTTP_LISTEN: ":8080"
+  GHP_SERVER_LISTEN: ":8080"
+  # Behind the TLS-terminating gateway: trust its X-Forwarded-* / Forwarded headers
+  # for scheme/host when generating absolute URLs (GHP_SERVER_BASE_URL is also set).
+  GHP_SERVER_TRUST_PROXY_HEADERS: "true"
   GHP_METRICS_LISTEN: ":9136"
   GHP_METRICS_ENABLED: "true"
   GHP_SERVER_BASE_URL: https://ghp.unkin.net

apps/base/ghp/deployment.yaml

@@ -102,7 +102,10 @@ spec:
             httpGet:
               path: /metrics
               port: metrics
-              scheme: HTTPS
+              # Plain HTTP: ghp only serves metrics over TLS in TLS mode
+              # (hasTLS). In reverse-proxy/plain mode the metrics server is
+              # cleartext, so probe with HTTP.
+              scheme: HTTP
             initialDelaySeconds: 30
             periodSeconds: 30
             successThreshold: 1
@@ -112,7 +115,10 @@ spec:
             httpGet:
               path: /metrics
               port: metrics
-              scheme: HTTPS
+              # Plain HTTP: ghp only serves metrics over TLS in TLS mode
+              # (hasTLS). In reverse-proxy/plain mode the metrics server is
+              # cleartext, so probe with HTTP.
+              scheme: HTTP
             initialDelaySeconds: 10
             periodSeconds: 5
             successThreshold: 1

apps/base/ghp/vmservicescrape.yaml

@@ -16,8 +16,6 @@ spec:
   endpoints:
     - port: metrics
       path: /metrics
-      scheme: https
-      # ghp serves metrics over TLS with an internal-CA cert; skip verification
-      # since the scrape targets a pod IP the cert SANs do not cover.
-      tlsConfig:
-        insecureSkipVerify: true
+      # ghp runs in plain reverse-proxy mode (no GHP_SERVER_HTTPS_LISTEN), so the
+      # metrics server is cleartext HTTP rather than TLS. Scrape over http.
+      scheme: http