fix(ci): isolate compose-idempotency network from archiv-net collisions

The name: archiv-net declaration (needed so docker-compose.observability.yml
can join the network as external: true) caused the compose-idempotency CI job
to collide with any archiv-net left on the runner from staging or a previous
run. mc would resolve 'minio' to the wrong container and fail with a signature
mismatch.

Make the network name interpolable via COMPOSE_NETWORK_NAME (default: archiv-net
so production/staging behaviour is unchanged). Inject COMPOSE_NETWORK_NAME=
test-idem-archiv-net into the stub env file so the idempotency test always
gets a fully isolated network.

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

.gitea/workflows/ci.yml

@@ -305,6 +305,7 @@ jobs:
           MAIL_PORT=1025
           APP_MAIL_FROM=noreply@local
           IMPORT_HOST_DIR=/tmp/dummy-import
+          COMPOSE_NETWORK_NAME=test-idem-archiv-net
           EOF
 
       - name: Bring up minio

docker-compose.prod.yml

@@ -39,7 +39,7 @@
 networks:
   archiv-net:
     driver: bridge
-    name: archiv-net
+    name: ${COMPOSE_NETWORK_NAME:-archiv-net}
 
 volumes:
   postgres-data: