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/vikunja/docker-compose.yml

@@ -1,5 +1,5 @@
 # Vikunja - Feladatkezelő listák és táblák (Todoist/Trello alternatíva)
-# Domain: tasks.${DOMAIN}
+# Domain: ${SUBDOMAIN}.${DOMAIN}
 # Database: None (file-based)
 # RAM: ~50M (mem_limit: 256M) | Pi-compatible: Yes
 #
@@ -14,7 +14,7 @@ services:
     restart: unless-stopped
     environment:
       - TZ=Europe/Budapest
-      - VIKUNJA_SERVICE_PUBLICURL=https://tasks.${DOMAIN}
+      - VIKUNJA_SERVICE_PUBLICURL=https://${SUBDOMAIN}.${DOMAIN}
       - VIKUNJA_SERVICE_JWTSECRET=${VIKUNJA_SERVICE_JWTSECRET}
     volumes:
       - vikunja_data:/app/vikunja/files
@@ -33,7 +33,7 @@ services:
       start_period: 30s
     labels:
       - "traefik.enable=true"
-      - "traefik.http.routers.vikunja.rule=Host(`tasks.${DOMAIN}`)"
+      - "traefik.http.routers.vikunja.rule=Host(`${SUBDOMAIN}.${DOMAIN}`)"
       - "traefik.http.routers.vikunja.entrypoints=websecure"
       - "traefik.http.routers.vikunja.tls=true"
       - "traefik.http.routers.vikunja.tls.certresolver=letsencrypt"