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-delegation-nginx.configmap.yaml

@@ -26,12 +26,12 @@ data:
         listen 8080;
 
         location /.well-known/matrix/server {
-          return 200 '{"m.server": "{{ .Values.ingress.domain }}:443"}';
+          return 200 '{"m.server": "{{ required "A Domain/Host must be provided (ingress.domain)" .Values.ingress.domain }}:443"}';
           add_header Content-Type application/json;
         }
       
         location /.well-known/matrix/client {
-          return 200 '{"m.homeserver": {"base_url": "https://{{ .Values.ingress.domain }}"}}';
+          return 200 '{"m.homeserver": {"base_url": "https://{{ required "A Domain/Host must be provided (ingress.domain)" .Values.ingress.domain }}"}}';
           add_header Content-Type application/json;
           add_header "Access-Control-Allow-Origin" *;
         }