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/opengist/docker-compose.yml

@@ -1,5 +1,5 @@
 # OpenGist - Kód snippetek megosztása (GitHub Gist alternatíva)
-# Domain: gist.${DOMAIN}
+# Domain: ${SUBDOMAIN}.${DOMAIN}
 # Database: None (file-based)
 # RAM: ~30M (mem_limit: 128M) | Pi-compatible: Yes
 #
@@ -13,7 +13,7 @@ services:
     restart: unless-stopped
     environment:
       - TZ=Europe/Budapest
-      - OG_EXTERNAL_URL=https://gist.${DOMAIN}
+      - OG_EXTERNAL_URL=https://${SUBDOMAIN}.${DOMAIN}
     volumes:
       - opengist_data:/opengist
     networks:
@@ -30,7 +30,7 @@ services:
       start_period: 30s
     labels:
       - "traefik.enable=true"
-      - "traefik.http.routers.opengist.rule=Host(`gist.${DOMAIN}`)"
+      - "traefik.http.routers.opengist.rule=Host(`${SUBDOMAIN}.${DOMAIN}`)"
       - "traefik.http.routers.opengist.entrypoints=websecure"
       - "traefik.http.routers.opengist.tls=true"
       - "traefik.http.routers.opengist.tls.certresolver=letsencrypt"