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/mealie/docker-compose.yml

@@ -1,5 +1,5 @@
 # Mealie - Recipe Manager & Meal Planner
-# Domain: recipes.${DOMAIN}
+# Domain: ${SUBDOMAIN}.${DOMAIN}
 # Database: None (SQLite, built-in)
 # RAM: ~200MB (mem_limit: 1000M) | Pi-compatible: Yes (arm64 only)
 #
@@ -25,7 +25,7 @@ services:
       - TZ=Europe/Budapest
       - MAX_WORKERS=1
       - WEB_CONCURRENCY=1
-      - BASE_URL=https://recipes.${DOMAIN}
+      - BASE_URL=https://${SUBDOMAIN}.${DOMAIN}
     volumes:
       - mealie_data:/app/data/
     networks:
@@ -42,7 +42,7 @@ services:
       start_period: 60s
     labels:
       - "traefik.enable=true"
-      - "traefik.http.routers.mealie.rule=Host(`recipes.${DOMAIN}`)"
+      - "traefik.http.routers.mealie.rule=Host(`${SUBDOMAIN}.${DOMAIN}`)"
       - "traefik.http.routers.mealie.entrypoints=websecure"
       - "traefik.http.routers.mealie.tls=true"
       - "traefik.http.routers.mealie.tls.certresolver=letsencrypt"