fix: revert service network refs to compose key name

Services should reference networks by their compose key name ('traefik'),
not by the env var. The actual Docker network name is already configurable
via TRAEFIK_NETWORK in the network definition at the bottom.

docker-compose.traefik.yml

@@ -73,7 +73,7 @@ services:
         condition: service_healthy
     networks:
       - backend
-      - ${TRAEFIK_NETWORK:-traefik}
+      - traefik
     restart: unless-stopped
     labels:
       - "traefik.enable=true"
@@ -97,7 +97,7 @@ services:
       - api
     networks:
       - backend
-      - ${TRAEFIK_NETWORK:-traefik}
+      - traefik
     restart: unless-stopped
     labels:
       - "traefik.enable=true"