feat(obs): make oauth2-proxy auth failures visible — alert on the gap, not on absent metrics

Jackett failed on 2026-09-30 in the most expensive way available: HTTP 200 and a
plausible page. oauth2-proxy answers an unauthenticated XHR with a 302 to Keycloak
rather than a 401; the browser follows it silently against a live SSO session and
the call resolves 200 carrying a 44 KB HTML dashboard where the app's JavaScript
wanted JSON. The user is never re-prompted, reloading never helps, and every uptime
check stays green. `jackett-down` watches replicas_available, which was 1 throughout.

The reason nothing alerted turns out not to be a missing rule. Measured on prod:

    count by (__name__) ({__name__=~"oauth2_proxy.*"})
      oauth2_proxy_requests_in_flight => 2

Two series, both from the SHARED forward-auth Deployment. The six per-app gate
sidecars (crafty, jackett, pihole, qbittorrent, radarr, sonarr) publish nothing at
all: no --metrics-address, so the metrics listener never starts; no `metrics`
container port; no ServiceMonitor or PodMonitor selects them. The alert the incident
calls for — sustained auth redirects to an authenticated user — is therefore NOT
WRITABLE, and writing it anyway would have shipped a rule that is present,
evaluating, permanently silent and indistinguishable from health.

So alert on the condition that made the incident invisible, from families that ARE
scraped:

  OAuth2ProxyGateUnobserved
    count by (namespace) (kube_pod_container_info{container="oauth2-proxy"})
      unless
    count by (namespace) (up{container="oauth2-proxy"})

kube_pod_container_info is kube-state-metrics (a declared scrape owner) and sees the
container whether or not it exports anything; `up` is UNIVERSAL and carries a
`container` label for any target, so the rule needs no advance knowledge of what job
name a future per-app scrape picks. Nothing in it touches oauth2_proxy_*, so it
cannot be inert for the reason it is reporting. Run verbatim against live prod it
returns six namespaces and correctly excludes oauth2-proxy. It clears per namespace
as each gate gains a real scrape.

Also:
- promtool companion test pinning all four states: fires unscraped, quiet when
  scraped, honours for:1h, and STOPS once a scrape appears under a different job name.
- runbooks/oauth2-proxy-auth-failures.md — tell the gate's refusal from the group
  check from the app's own login from an XHR that silently got HTML, with the
  2026-09-30 jackett trace as the worked example and the Loki queries to use while
  there are no metrics. Registered in the configMapGenerator and README.
- runbooks/README.md gains an inert-alerts row for the unscraped sidecars.

Rejected: an alert on the shared proxy's oauth2_proxy_requests_total ratio. It needs
a conformance/ SCRAPE_OWNERS entry (human approval) and, decisively, would not have
fired on this incident — the shared instance is not in any sidecar-gated app's data
path and its current pods have proxied nothing. It would have looked like coverage
and been none.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

platform/oauth2-proxy/README.md

@@ -88,3 +88,39 @@ This proves the R-IDP-02 property (unauthenticated cannot reach the upstream) on
 3. Log in as a **non-platform-admins** user -> expect **403** (authenticated but not authorized).
 4. Confirm the session cookie is `.home.keithjackson.dev`-scoped (single sign-on across the other
    protected UIs).
+
+## Observability: what this component can and cannot see (measured 2026-09-30)
+
+`servicemonitor.yaml` scrapes **this namespace's** deployment and nothing else. On prod that is
+two targets, `job="oauth2-proxy"`, both `up=1`. The complete set of oauth2-proxy-specific series
+in the TSDB is:
+
+    count by (__name__) ({__name__=~"oauth2_proxy.*"})
+      oauth2_proxy_requests_in_flight => 2
+
+**The per-app gate sidecars are not scraped, and there is nothing to scrape.** Six apps run an
+`oauth2-proxy` container of their own — crafty, jackett, pihole, qbittorrent, radarr, sonarr —
+each with `--http-address=0.0.0.0:4180` and **no `--metrics-address`**, so oauth2-proxy's metrics
+listener never starts. They declare only a `http:4180` container port, no `metrics` port exists on
+their Services, and `kubectl get servicemonitors,podmonitors -A` selects none of them. Any alert
+written against `oauth2_proxy_requests_total{namespace="jackett"}` would be structurally inert.
+
+This matters because the gate's characteristic failure is invisible by construction: an
+unauthenticated **XHR** gets a `302` to Keycloak rather than a `401`, the browser follows it
+against a live SSO session with no prompt, and the call resolves **200 carrying an HTML page**
+where the app expected JSON. Nothing in the request path reports an error. On 2026-09-30 jackett
+spent a burst of requests in exactly that state, re-established a session, and lost it 26 seconds
+into a 5-minute session, while `jackett-down` sat at 1 replica available the whole time.
+
+`auth-visibility-alerts.yaml` therefore alerts on **the gap**, not on the incident:
+`OAuth2ProxyGateUnobserved` compares `kube_pod_container_info{container="oauth2-proxy"}` against
+`up{container="oauth2-proxy"}` — two already-scraped families — and names every namespace running
+a gate that nothing observes. It fires for six namespaces today and clears per-namespace as each
+one gains a scrape. `runbooks/oauth2-proxy-auth-failures.md` is the by-hand substitute meanwhile,
+and it reads the sidecar logs out of Loki (Alloy collects them; 72h retention).
+
+**To close the gap for an app**, three things are needed together, on that app's own manifest:
+`--metrics-address=0.0.0.0:44180` on the sidecar, a `metrics` containerPort plus the matching
+Service port, and a ServiceMonitor selecting it. Only then does the auth-churn rule — sustained
+`code="302"` as a share of `oauth2_proxy_requests_total`, held across a window longer than the
+session lifetime — become writable rather than decorative.