Reserve fixed IPs on the lava network so Caddy can always start

lava was created by hand without an ip_range, so any container could be
given Caddy's fixed IP (172.20.0.5). After a reboot the containers race
for the low addresses and Caddy often fails with "Address already in use".

- The docker role now creates lava as 172.20.0.0/16 with dynamic
  addresses limited to 172.20.128.0/17, so 172.20.0.x is free for fixed
  IPs, and warns if the network still has the old layout.
- Gitea gets fixed IP 172.20.0.6 once the network has the reserved range,
  since its ufw-docker SSH rule points at its IP.
- ufw-docker allow rules for Caddy (80, 443) and Gitea SSH are managed by
  the playbook and follow IP changes.
- migrate-lava-network.yml recreates the existing network once: it takes
  down every compose project on lava (volumes kept), recreates the
  network, and brings them back up, Caddy first.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

group_vars/all/infrastructure.yml

@@ -4,7 +4,13 @@
 docker:
   network_name: "lava"
   stacks_path: "/opt/stacks"
-  hairpin_ip: "172.20.0.5"
+  hairpin_ip: "172.20.0.5"   # Caddy's fixed IP
+  # Shared network layout. Docker only hands out addresses from ip_range,
+  # so 172.20.0.x is kept free for fixed IPs and can't be taken by others.
+  subnet: "172.20.0.0/16"
+  gateway: "172.20.0.1"
+  ip_range: "172.20.128.0/17"
+  gitea_ip: "172.20.0.6"     # fixed so the ufw-docker SSH rule stays valid
 
 # SMTP configuration
 smtp: