fix: declare cell-network external; pre-create in Makefile start/update

Docker Compose v5 enforces label ownership on networks it creates. On
systems where cell-network was created by an older compose version (no
labels), Caddy and other services fail to start with "incorrect label"
error.

Declaring the network external in docker-compose.yml skips label
validation. The Makefile start/update targets now create the network if
it doesn't exist (idempotent). The reinstall and uninstall (full) paths
explicitly delete the network so fresh recreations are clean.

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

docker-compose.yml

@@ -181,7 +181,4 @@ services:
 networks:
   cell-network:
     name: cell-network
-    driver: bridge
-    ipam:
-      config:
-        - subnet: ${CELL_NETWORK:-172.20.0.0/16}
+    external: true