fix: add missing swarm deploy block to mysql backup service

- Docker Swarm (Portainer) ignores 'restart: always' and often requires a 'deploy' block with replica counts to properly schedule a new service.

- Replaced standard docker-compose restart policy with a Swarm-compliant deploy block.

docker-compose.yml

@@ -45,7 +45,10 @@ services:
 
   backup-mysql:
     image: fradelg/mysql-cron-backup
-    restart: always
+    deploy:
+      replicas: 1
+      restart_policy:
+        condition: on-failure
     environment:
       MYSQL_HOST: db
       MYSQL_USER: root