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>

nginx/nginx.conf

@@ -1,3 +1,5 @@
+# Generated from nginx.conf.template — do not edit directly.
+# Regenerate: DOMAIN=yourdomain.com envsubst '${DOMAIN}' < nginx/nginx.conf.template > nginx/nginx.conf
 upstream web {
     server 127.0.0.1:8080;
 }