voucher-vault: strict values.schema.json and null-safe hardening

No ingress gate: this chart already gates its Ingress correctly. Two
changes plus one guard that the existing gate turned out to need.

1. values.schema.json (#68): additionalProperties: false on every
   chart-owned object level. Lists that go verbatim into the pod spec
   (ports, storage.volumes, storage.volumeMounts) stay open.

2. Null-safety for hardening, probes and resources (#99). Two specifics
   of this chart:

   - The probes were wrapped in "if" guards, so an erased probe block did
     not render "livenessProbe: null" - it removed the probe entirely and
     silently. The guards are gone; an empty block now means chart
     defaults, so the workload keeps its probes.
   - The rebuilt probes keep the Host: 127.0.0.1 header. Django's
     ALLOWED_HOSTS only accepts 127.0.0.1 and the configured DOMAIN, so a
     probe without it would get 400 and the pod would never be ready.

3. ingress.domain is required() where it is read. The dependency lives in
   values.yaml, not in a template: the DOMAIN env entry is a tpl string
   feeding Django's ALLOWED_HOSTS. With the existing gate and no domain
   the app rendered DOMAIN: "" and answered 400 to every real request -
   silently, and already before this change. It now fails loudly.

Apart from an added comment, the rendered manifest is unchanged.

voucher-vault/values.yaml

@@ -86,8 +86,12 @@ voucherVault:
       protocol: TCP
       name: http
   env:
+    # ingress.domain is not a pure Ingress value: Django's ALLOWED_HOSTS only
+    # accepts 127.0.0.1 and this DOMAIN, so an empty value makes the app answer
+    # 400 to every real request. It therefore stays required even when the
+    # Ingress is switched off via ingress.enabled.
     - name: DOMAIN
-      value: "{{ .Values.ingress.domain }}"
+      value: '{{ required "A Domain/Host must be provided (ingress.domain)" .Values.ingress.domain }}'
     - name: SECURE_COOKIES
       value: "True"
     - name: SESSION_COOKIE_AGE
@@ -196,6 +200,9 @@ storage:
   storageClass: null
 
 ingress:
+  # Set to false to omit the Ingress entirely. Note that ingress.domain stays
+  # required even then: it is also Django's ALLOWED_HOSTS entry (the DOMAIN env
+  # above), not just the Ingress host. Only clusterIssuer becomes unnecessary.
   enabled: true
   clusterIssuer: null
   domain: null