Make delegation nginx image configurable in matrix-synapse

The delegation nginx image was hardcoded as nginx:1.28.3 in the
deployment template. It is now rendered from the new values
matrix.delegation.image.repository / matrix.delegation.image.tag,
defaulting to nginx:1.30.4 (current stable line, tag verified against
Docker Hub). README documents that auxiliary/sidecar images are pinned
via values and reviewed with every app-version update round.

Chart version: 4.1.1+upv1.159.0 -> 4.2.0+upv1.159.0 (minor: additive
values change).

matrix-synapse/values.yaml

@@ -17,6 +17,11 @@ matrix:
     replicas: 1
     enabled: false
     serverUrl: null
+    # Auxiliary image, pinned independently of appVersion (which tracks
+    # Synapse). Reviewed alongside every app-version update round.
+    image:
+      repository: nginx
+      tag: 1.30.4
     # The delegation nginx runs as the image's unprivileged nginx user
     # (uid 101) and listens on the unprivileged port 8080; pid file and temp
     # paths are redirected to the /tmp emptyDir by the mounted nginx.conf.