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/nextcloud/docker-compose.yml

@@ -1,5 +1,5 @@
 # Nextcloud - Saját felhő tárhely - Google Drive/Dropbox alternatíva
-# Domain: cloud.${DOMAIN}
+# Domain: ${SUBDOMAIN}.${DOMAIN}
 # Database: mariadb
 # RAM: ~256M (mem_limit: 1024M) | Pi-compatible: No
 #
@@ -29,9 +29,9 @@ services:
       - MYSQL_HOST=nextcloud-db
       - NEXTCLOUD_ADMIN_USER=${NEXTCLOUD_ADMIN_USER:-admin}
       - NEXTCLOUD_ADMIN_PASSWORD=${NEXTCLOUD_ADMIN_PASSWORD}
-      - NEXTCLOUD_TRUSTED_DOMAINS=cloud.${DOMAIN}
+      - NEXTCLOUD_TRUSTED_DOMAINS=${SUBDOMAIN}.${DOMAIN}
       - OVERWRITEPROTOCOL=https
-      - OVERWRITEHOST=cloud.${DOMAIN}
+      - OVERWRITEHOST=${SUBDOMAIN}.${DOMAIN}
       - REDIS_HOST=nextcloud-redis
     volumes:
       - nextcloud_html:/var/www/html
@@ -51,7 +51,7 @@ services:
       start_period: 30s
     labels:
       - "traefik.enable=true"
-      - "traefik.http.routers.nextcloud.rule=Host(`cloud.${DOMAIN}`)"
+      - "traefik.http.routers.nextcloud.rule=Host(`${SUBDOMAIN}.${DOMAIN}`)"
       - "traefik.http.routers.nextcloud.entrypoints=websecure"
       - "traefik.http.routers.nextcloud.tls=true"
       - "traefik.http.routers.nextcloud.tls.certresolver=letsencrypt"