🔧(backend) add Docker network for shared Keycloak OIDC authentication

Define Docker network enabling external service providers to share Keycloak
instance with local development stack, supporting OIDC authentication flow
where services obtain tokens from shared Keycloak then pass to Meet
for introspection and validation.

Prepares Meet infrastructure for multi-service authentication architecture
though external service provider Docker Compose integration changes remain
in separate repository.

compose.yml

@@ -90,6 +90,9 @@ services:
         - createwebhook
     extra_hosts:
       - "127.0.0.1.nip.io:host-gateway"
+    networks:
+      - resource-server
+      - default
   
   celery-dev:
     user: ${DOCKER_USER:-1000}
@@ -145,6 +148,9 @@ services:
       - ./docker/files/etc/nginx/conf.d:/etc/nginx/conf.d:ro
     depends_on:
       - keycloak
+    networks:
+      - resource-server
+      - default
 
   frontend:
     user: "${DOCKER_USER:-1000}"
@@ -298,3 +304,7 @@ services:
       watch:
         - action: rebuild
           path: ./src/summary
+
+networks:
+  default:
+  resource-server: