config: make domain configurable via DOMAIN env var

Replace hardcoded evosync.ru with a DOMAIN variable read from .env.
nginx.conf is now generated from nginx.conf.template via envsubst;
init-letsencrypt.sh reads DOMAIN from .env and fails loudly if unset.
README documents the new variable and first-deploy TLS workflow.

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

docker-compose.yml

@@ -39,7 +39,7 @@ services:
       DATABASE_URL: mysql+pymysql://${DB_USER}:${DB_PASSWORD}@db:3306/${DB_NAME}
       REDIS_URL: redis://redis:6379/0
       SECRET_KEY: ${SECRET_KEY:-change-me-in-production}
-      BASE_URL: ${BASE_URL:-https://evosync.ru}
+      BASE_URL: ${BASE_URL:-https://${DOMAIN}}
       EVOTOR_APP_ID: ${EVOTOR_APP_ID:-}
       EVOTOR_WEBHOOK_SECRET: ${EVOTOR_WEBHOOK_SECRET:-}
       JIVOSITE_WIDGET_ID: ${JIVOSITE_WIDGET_ID:-}