fix: use fixed IP for wzp-relay (wzp-web can't resolve hostnames)

wzp-web --relay only accepts IP:port, not Docker hostnames.
Fixed IP 172.28.0.10 on backend network with explicit subnet.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

warzone/deploy/docker/docker-compose.yml

@@ -65,7 +65,8 @@ services:
       - "--auth-url"
       - "http://warzone-server:7700/v1/auth/validate"
     networks:
-      - backend
+      backend:
+        ipv4_address: 172.28.0.10
 
   # ─── WZP web bridge (browser WS ↔ QUIC relay) ───
   wzp-web:
@@ -78,7 +79,7 @@ services:
       - "--port"
       - "8080"
       - "--relay"
-      - "wzp-relay:4433"
+      - "172.28.0.10:4433"
       - "--auth-url"
       - "http://warzone-server:7700/v1/auth/validate"
     depends_on:
@@ -99,3 +100,6 @@ volumes:
 networks:
   frontend:
   backend:
+    ipam:
+      config:
+        - subnet: 172.28.0.0/24