fix(helm): hardening from review

- _helpers.tpl: required guard on image.tag — silent empty deploys can
  no longer happen; helm fails fast with a clear message.
- configmap-nginx: HTTPS fastcgi param uses if_not_empty, so PHP only
  sees HTTPS when X-Forwarded-Proto is actually present.
- deployment-app: add startupProbe with 100s budget so first-boot cache
  warmup doesn't trip liveness.
- deployment-horizon: failureThreshold=5 on the horizon:status probe;
  transient Valkey blips no longer cause restart loops.
- job-migrate: mount oauth-keys so seeders that touch Passport clients
  don't silently fail.
- statefulset-valkey: replace separate password Secret with a
  requirePassword toggle that reads REDIS_PASSWORD from the main chart
  Secret (same Secret app/horizon/scheduler already mount). Liveness
  probe authenticates with the password when set.
- values-us-prod: enable valkey.requirePassword.
- README: add REDIS_PASSWORD to bootstrap procedure.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

helm/ezscale-website/values-us-prod.yaml

@@ -43,9 +43,11 @@ mariadb:
   database: ezscale_billing
   username: ezscale_billing_app
 
-# Per-app Valkey for sessions/cache/queue.
+# Per-app Valkey for sessions/cache/queue. requirePassword=true means
+# REDIS_PASSWORD must be present in ezscale-website-secrets.
 valkey:
   enabled: true
+  requirePassword: true
   storage:
     size: 10Gi
     storageClassName: longhorn