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/neo4j/docker-compose.yml.j2

@@ -1,6 +1,6 @@
 services:
   neo4j:
-    image: neo4j:{{neo4j_version}}
+    image: neo4j:{{neo4j_image_version}}
     container_name: neo4j
     restart: unless-stopped
     ports: