chore(images): pin service image updates

Replace floating service images with reviewable tags or digests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

edu_master/k8s/webinar-checker.yaml

@@ -19,7 +19,7 @@ spec:
       # redis healthy -> session-keeper healthy (EXISTS EDU_PHPSESSID) -> playwright started
       initContainers:
         - name: wait-deps
-          image: redis:alpine
+          image: redis:8.0.3-alpine
           command:
             - /bin/sh
             - -ec