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>

.env.example

@@ -6,7 +6,8 @@ DB_PASSWORD=evosync
 
 # App
 SECRET_KEY=change-me-in-production
-BASE_URL=https://evosync.ru
+DOMAIN=yourdomain.com
+BASE_URL=https://${DOMAIN}
 
 # Evotor
 EVOTOR_APP_ID=