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/Chart.yaml

@@ -5,7 +5,7 @@ icon: https://raw.githubusercontent.com/l4rm4nd/VoucherVault/main/myapp/static/a
 
 type: application
 
-version: 4.0.0+up1.30.1
+version: 5.0.0+up1.30.1
 appVersion: "1.30.1"
 
 maintainers: