matrix-synapse: strict schema, ingress gate and null-safe hardening

Three changes, across both workloads (Synapse StatefulSet and delegation
nginx Deployment).

1. values.schema.json (#68): additionalProperties: false on every
   chart-owned object level, including the whole matrix.delegation block
   with its own contexts, probes and resources.

2. Ingress gate (#93): the Ingress template is wrapped in
   {{- if .Values.ingress.enabled }} and the key is added to values.yaml
   with default true, so behaviour is unchanged.

   The domain checkpoint found a second reader: the delegation nginx
   serves ingress.domain in its /.well-known/matrix/{server,client}
   endpoints, and read it without a guard. Gating alone would therefore
   have published '{"m.server": ":443"}' and base_url "https://" - a
   federation-breaking answer that renders and starts cleanly. Both reads
   are now required(), so that case fails loudly. Only clusterIssuer
   actually becomes unnecessary when the Ingress is off.

3. Null-safety for hardening, probes and resources (#99) for both
   workloads. The rebuilt probes keep each workload's own endpoint:
   Synapse /health, delegation /.well-known/matrix/server - the endpoint
   that workload exists to serve - including its shorter 3s timeout. The
   delegation pod context keeps having no fsGroup, because it mounts no
   volume that needs one.

The nginx checksum/config annotation from #82 is untouched.

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

Rendering is byte-identical to 6.0.0, both with the CI values (which
enable delegation) and with the production bundle values.

matrix-synapse/templates/matrix-synapse.sfs.yaml

@@ -18,12 +18,12 @@ spec:
     spec:
       automountServiceAccountToken: false
       securityContext:
-        {{- toYaml .Values.matrix.securityContext.pod | nindent 8 }}
+        {{- include "matrix-synapse.podSecurityContext" .Values.matrix.securityContext | nindent 8 }}
       containers:
         - name: "{{ .Release.Name }}-{{ .Chart.Name }}-container"
           image: "ghcr.io/element-hq/synapse:{{ .Chart.AppVersion }}"
           securityContext:
-            {{- toYaml .Values.matrix.securityContext.container | nindent 12 }}
+            {{- include "matrix-synapse.containerSecurityContext" .Values.matrix.securityContext | nindent 12 }}
           env:
             - name: SYNAPSE_SERVER_NAME
               value: "{{ required "You must provide a Server-Name" .Values.matrix.serverName }}"
@@ -56,13 +56,13 @@ spec:
               containerPort: 8008
               protocol: TCP
           readinessProbe:
-            {{- toYaml .Values.matrix.readinessProbe | nindent 12 }}
+            {{- include "matrix-synapse.readinessProbe" .Values.matrix.readinessProbe | nindent 12 }}
           livenessProbe:
-            {{- toYaml .Values.matrix.livenessProbe | nindent 12 }}
+            {{- include "matrix-synapse.livenessProbe" .Values.matrix.livenessProbe | nindent 12 }}
           startupProbe:
-            {{- toYaml .Values.matrix.startupProbe | nindent 12 }}
+            {{- include "matrix-synapse.startupProbe" .Values.matrix.startupProbe | nindent 12 }}
           resources:
-            {{- include "matrix-synapse.resources" .Values.matrix.resources | nindent 12 }}
+            {{- include "matrix-synapse.effectiveResources" .Values.matrix.resources | nindent 12 }}
           volumeMounts:
             - name: matrix-synapse-volume
               mountPath: /data