Fix image tags that failed to pull on test-cloud

- discourse: 2026.10.0 -> 2026.10.0-latest (correct tag format)
- formbricks: Docker Hub 6.1.0 -> ghcr.io 4.6.0 (moved to GHCR)
- zulip: Docker Hub 12.3-0 -> ghcr.io 12.3-0 (12.x on GHCR)
- open-webui: v0.12.3 -> v0.12.4 (newer release)

discourse/versions/2026/deployment.yaml

@@ -23,7 +23,7 @@ spec:
         fsGroupChangePolicy: Always
       initContainers:
         - name: discourse-migrate
-          image: discourse/discourse:2026.10.0
+          image: discourse/discourse:2026.10.0-latest
           imagePullPolicy: "IfNotPresent"
           securityContext:
             allowPrivilegeEscalation: false
@@ -87,7 +87,7 @@ spec:
               mountPath: /shared
       containers:
         - name: discourse
-          image: discourse/discourse:2026.10.0
+          image: discourse/discourse:2026.10.0-latest
           imagePullPolicy: "IfNotPresent"
           command:
             - /sbin/boot
@@ -192,7 +192,7 @@ spec:
             - name: discourse-data
               mountPath: /shared
         - name: sidekiq
-          image: discourse/discourse:2026.10.0
+          image: discourse/discourse:2026.10.0-latest
           imagePullPolicy: "IfNotPresent"
           securityContext:
             allowPrivilegeEscalation: false