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/n8n/docker-compose.yml

@@ -1,5 +1,5 @@
 # n8n - Workflow automatizálás vizuális szerkesztővel
-# Domain: auto.${DOMAIN}
+# Domain: ${SUBDOMAIN}.${DOMAIN}
 # Database: None (file-based)
 # RAM: ~150M (mem_limit: 512M) | Pi-compatible: No
 #
@@ -14,9 +14,9 @@ services:
     restart: unless-stopped
     environment:
       - TZ=Europe/Budapest
-      - N8N_HOST=auto.${DOMAIN}
+      - N8N_HOST=${SUBDOMAIN}.${DOMAIN}
       - N8N_PROTOCOL=https
-      - WEBHOOK_URL=https://auto.${DOMAIN}/
+      - WEBHOOK_URL=https://${SUBDOMAIN}.${DOMAIN}/
       - N8N_ENCRYPTION_KEY=${N8N_ENCRYPTION_KEY}
     volumes:
       - n8n_data:/home/node/.n8n
@@ -34,7 +34,7 @@ services:
       start_period: 30s
     labels:
       - "traefik.enable=true"
-      - "traefik.http.routers.n8n.rule=Host(`auto.${DOMAIN}`)"
+      - "traefik.http.routers.n8n.rule=Host(`${SUBDOMAIN}.${DOMAIN}`)"
       - "traefik.http.routers.n8n.entrypoints=websecure"
       - "traefik.http.routers.n8n.tls=true"
       - "traefik.http.routers.n8n.tls.certresolver=letsencrypt"