chore(ansible): centralize third-party Docker image versions

Add centralized image version variables in group_vars/all/vars.yml for
vulnerability tracking and controlled upgrades of third-party Docker
images (casdoor, flower, grafana-mcp, gitea-mcp, neo4j, memcached,
nginx, oauth2-proxy, rabbitmq, searxng).

Update vault.yml accordingly.

ansible/inventory/hosts

@@ -17,6 +17,7 @@ ubuntu:
     rosalind.incus:
     sycorax.incus:
     titania.incus:
+    umbriel.incus:
 
 # Service-specific groups for targeted deployments
 agent_s: