feat: use ${SUBDOMAIN} variable in all templates

All 51 docker-compose.yml: replaced hardcoded subdomain.${DOMAIN}
with ${SUBDOMAIN}.${DOMAIN} in Traefik labels, app env vars, and
comments.

All 51 .felhom.yml: added SUBDOMAIN deploy field (type: subdomain)
with default matching existing subdomain metadata value.

Works with felhom-controller v0.27.0 which validates and stores the
user-chosen subdomain in app.yaml. Existing deployed apps get
SUBDOMAIN auto-injected via InjectMissingFields() on next sync.

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

templates/romm/docker-compose.yml

@@ -1,5 +1,5 @@
 # ROMM - ROM Manager for Game Libraries
-# Domain: arcade.${DOMAIN}
+# Domain: ${SUBDOMAIN}.${DOMAIN}
 # Database: MariaDB + Redis
 # RAM: ~300MB (mem_limit: 1024M total — romm 512M + mariadb 384M + redis 128M) | Pi-compatible: No (MariaDB + heavy)
 #
@@ -80,7 +80,7 @@ services:
       start_period: 60s
     labels:
       - "traefik.enable=true"
-      - "traefik.http.routers.romm.rule=Host(`arcade.${DOMAIN}`)"
+      - "traefik.http.routers.romm.rule=Host(`${SUBDOMAIN}.${DOMAIN}`)"
       - "traefik.http.routers.romm.entrypoints=websecure"
       - "traefik.http.routers.romm.tls=true"
       - "traefik.http.routers.romm.tls.certresolver=letsencrypt"