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/bookstack/docker-compose.yml

@@ -1,5 +1,5 @@
 # BookStack - Egyszerű, könyv-szerű wiki és dokumentáció platform
-# Domain: wiki.${DOMAIN}
+# Domain: ${SUBDOMAIN}.${DOMAIN}
 # Database: mariadb
 # RAM: ~150M (mem_limit: 512M) | Pi-compatible: Yes
 #
@@ -26,7 +26,7 @@ services:
       - DB_PASSWORD=${DB_PASSWORD}
       - DB_DATABASE=bookstack
       - APP_KEY=${APP_KEY}
-      - APP_URL=https://wiki.${DOMAIN}
+      - APP_URL=https://${SUBDOMAIN}.${DOMAIN}
     volumes:
       - bookstack_config:/config
     networks:
@@ -44,7 +44,7 @@ services:
       start_period: 30s
     labels:
       - "traefik.enable=true"
-      - "traefik.http.routers.bookstack.rule=Host(`wiki.${DOMAIN}`)"
+      - "traefik.http.routers.bookstack.rule=Host(`${SUBDOMAIN}.${DOMAIN}`)"
       - "traefik.http.routers.bookstack.entrypoints=websecure"
       - "traefik.http.routers.bookstack.tls=true"
       - "traefik.http.routers.bookstack.tls.certresolver=letsencrypt"