feat(cloudflared): añadir stack de Cloudflare Tunnel + gestión DNS

- docker-compose.yml: tunnel de Cloudflare para exponer servicios sin
  abrir puertos en el router
- add-domain.sh: script para añadir subdominios al tunnel y crear
  registros DNS en Cloudflare vía API
- stack.env: plantilla con tokens y configuración del tunnel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cloudflared/docker-compose.yml

@@ -0,0 +1,20 @@
+services:
+  cloudflared:
+    image: ${CLOUDFLARED_IMAGE}
+    container_name: cloudflared
+    restart: unless-stopped
+    command: tunnel --no-autoupdate run
+    environment:
+      TUNNEL_TOKEN: ${CLOUDFLARE_TUNNEL_TOKEN}
+      TZ: ${TZ:-America/Mexico_City}
+    networks:
+      - proxy
+    healthcheck:
+      test: ["CMD", "cloudflared", "tunnel", "info"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+
+networks:
+  proxy:
+    external: true