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/adventurelog/docker-compose.yml

@@ -1,5 +1,5 @@
 # AdventureLog - Utazási napló és kalandtervező
-# Domain: travel.${DOMAIN}
+# Domain: ${SUBDOMAIN}.${DOMAIN}
 # Database: postgres
 # RAM: ~100M (mem_limit: 384M) | Pi-compatible: Yes
 #
@@ -22,9 +22,9 @@ services:
       - PGDATABASE=adventurelog
       - PGUSER=adventurelog
       - PGPASSWORD=${DB_PASSWORD}
-      - PUBLIC_URL=https://travel.${DOMAIN}
-      - CSRF_TRUSTED_ORIGINS=https://travel.${DOMAIN}
-      - FRONTEND_URL=https://travel.${DOMAIN}
+      - PUBLIC_URL=https://${SUBDOMAIN}.${DOMAIN}
+      - CSRF_TRUSTED_ORIGINS=https://${SUBDOMAIN}.${DOMAIN}
+      - FRONTEND_URL=https://${SUBDOMAIN}.${DOMAIN}
     volumes:
       - adventurelog_media:/code/media
     networks:
@@ -70,13 +70,13 @@ services:
     environment:
       - PUBLIC_SERVER_URL=http://adventurelog:8000
       - BODY_SIZE_LIMIT=Infinity
-      - ORIGIN=https://travel.${DOMAIN}
+      - ORIGIN=https://${SUBDOMAIN}.${DOMAIN}
     networks:
       - traefik-public
       - adventurelog-internal
     labels:
       - "traefik.enable=true"
-      - "traefik.http.routers.adventurelog.rule=Host(`travel.${DOMAIN}`)"
+      - "traefik.http.routers.adventurelog.rule=Host(`${SUBDOMAIN}.${DOMAIN}`)"
       - "traefik.http.routers.adventurelog.entrypoints=websecure"
       - "traefik.http.routers.adventurelog.tls=true"
       - "traefik.http.routers.adventurelog.tls.certresolver=letsencrypt"