outline: strict values.schema.json and null-safe hardening

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

1. values.schema.json (#68): additionalProperties: false on every
   chart-owned object level.

2. Null-safety for hardening, probes and resources (#99). All three
   probes keep /_health when rebuilt: it is Outline's only health
   endpoint, and "/" returns 500 as well when the database is down, so
   there is no dependency-free path to fall back to.

3. ingress.domain is required() where the StatefulSet reads it. It feeds
   the URL env var - Outline's public URL for generated links and the
   OIDC redirect URI - and did so without a guard. With the existing gate
   and no domain it rendered "https://", breaking logins silently. That
   hole predates this change; it now fails loudly.

outline.storage stays deliberately not null-safe: an erased block yields
an invalid PVC, which is the loud class #99 excludes.

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

outline/Chart.yaml

@@ -5,7 +5,7 @@ icon: https://raw.githubusercontent.com/outline/outline/main/public/images/icon-
 
 type: application
 
-version: 5.0.0+up1.9.2
+version: 6.0.0+up1.9.2
 appVersion: 1.9.2
 
 maintainers: