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/ghost/docker-compose.yml

@@ -1,5 +1,5 @@
 # Ghost - Professzionális blog és hírlevél platform
-# Domain: blog.${DOMAIN}
+# Domain: ${SUBDOMAIN}.${DOMAIN}
 # Database: None (file-based)
 # RAM: ~150M (mem_limit: 512M) | Pi-compatible: No
 #
@@ -14,7 +14,7 @@ services:
     environment:
       - TZ=Europe/Budapest
       - NODE_ENV=production
-      - url=https://blog.${DOMAIN}
+      - url=https://${SUBDOMAIN}.${DOMAIN}
       - database__client=sqlite3
       - database__connection__filename=content/data/ghost.db
     volumes:
@@ -33,7 +33,7 @@ services:
       start_period: 30s
     labels:
       - "traefik.enable=true"
-      - "traefik.http.routers.ghost.rule=Host(`blog.${DOMAIN}`)"
+      - "traefik.http.routers.ghost.rule=Host(`${SUBDOMAIN}.${DOMAIN}`)"
       - "traefik.http.routers.ghost.entrypoints=websecure"
       - "traefik.http.routers.ghost.tls=true"
       - "traefik.http.routers.ghost.tls.certresolver=letsencrypt"