fix: force Traefik to use domverse network for routing

When a container is attached to multiple networks, Traefik picks an IP
from any of them. The frontend is on both domverse and flight-radar_default;
Traefik isn't on the latter, so ~50% of requests 504 after 30s.

Adding traefik.docker.network=domverse pins routing to the shared network.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flight-comparator/docker-compose.yml

@@ -31,6 +31,7 @@ services:
       - domverse  # Traefik discovers the container on this network
     labels:
       # Traefik routing
+      - "traefik.docker.network=domverse"
       - "traefik.enable=true"
       - "traefik.http.routers.flight-radar.rule=Host(`flights.domverse-berlin.eu`)"
       - "traefik.http.routers.flight-radar.entrypoints=https"