fix(infrastructure): use the actual Traefik network name (proxy)

The external network on the target server is named "proxy", not
traefik-network as originally assumed - deploy was failing with
"network traefik-network not found". Updated the compose file and
both the infra README and vault deployment doc to match.

infrastructure/docker-compose.yml

@@ -52,7 +52,7 @@ services:
       start_period: 30s
     networks:
       - app-network
-      - traefik-network
+      - proxy
 
   worker:
     image: git.devils.zone/devil/youtube-studio-flow-backend:${IMAGE_TAG:-latest}
@@ -99,7 +99,7 @@ services:
         condition: service_healthy
     networks:
       - app-network
-      - traefik-network
+      - proxy
 
   postgres:
     image: postgres:16-alpine
@@ -146,5 +146,5 @@ volumes:
 networks:
   app-network:
     driver: bridge
-  traefik-network:
+  proxy:
     external: true