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/templates/job-migrate.yaml

@@ -41,4 +41,18 @@ spec:
                 name: {{ include "ezscale-website.fullname" . }}-env
             - secretRef:
                 name: {{ include "ezscale-website.secretName" . }}
+          volumeMounts:
+            - name: oauth-keys
+              mountPath: /var/www/html/secrets
+              readOnly: true
+      volumes:
+        - name: oauth-keys
+          secret:
+            secretName: {{ include "ezscale-website.secretName" . }}
+            items:
+              - key: oauth-private.key
+                path: oauth-private.key
+              - key: oauth-public.key
+                path: oauth-public.key
+            optional: true
 {{- end }}