Guard the four unguarded OnePasswordItem references with required()

calibre-web-automated 7.0.0, minecraft-server 6.0.0 and urlaubsverwaltung
10.0.0 each majors: an installation that renders today with an empty itemPath
stops rendering. searxng 1.0.6 is documentation only - its valkey reference is
correctly optional and stays gated by {{- if }}.

Every message names the values path. values.yaml now says per key why it is
required, reusing the wording the charts' NOTES.txt already carry. The
hand-rolled "cwaSecretRef IS UNSET" block in calibre-web-automated's NOTES.txt
is gone: after the guard the render aborts first and that block is
unreachable. minecraft-server's note stays true and now points at the guard.

No "required" array in the schemas, and README guideline 12 now says why. The
plan called for one; measuring it showed it could never fire: the key ships as
null, so it is present at validation time, and JSON Schema's required tests
presence, not null-ness. Making it fire means either dropping the null default
or narrowing the type to "string", and both turn helm lint --strict red on the
chart's own defaults. The schema therefore describes the key as required and
required() enforces it. The README bullet on how null reaches the schema was
also wrong in the case it was being applied to - a null chart default is not
coalesced away - and now distinguishes the three cases.

searxng: the claim "charts in this repo have no dependencies (README,
guideline 13)" in the valkey template, in values.yaml and twice in _helpers.tpl
has been false since #214 and cited the wrong guideline; dependencies are
admissible and are guideline 14. The same sentence survives in other charts'
helpers and needs its own round.

urlaubsverwaltung/templates/urlaubsverwaltung-db.secret.yaml

@@ -3,4 +3,4 @@ kind: OnePasswordItem
 metadata:
   name: {{ .Release.Name }}-{{ .Chart.Name }}-db-secret
 spec:
-  itemPath: {{ .Values.secrets.databaseSecretRef }}
+  itemPath: {{ required "A database Secret Reference must be provided (secrets.databaseSecretRef)" .Values.secrets.databaseSecretRef }}

urlaubsverwaltung/templates/urlaubsverwaltung-oidc.secret.yaml

@@ -3,4 +3,4 @@ kind: OnePasswordItem
 metadata:
   name: {{ .Release.Name }}-{{ .Chart.Name }}-oidc-secret
 spec:
-  itemPath: {{ .Values.secrets.oidcSecretRef }}
+  itemPath: {{ required "An OIDC Secret Reference must be provided (secrets.oidcSecretRef)" .Values.secrets.oidcSecretRef }}