docs: native Ubuntu install as primary; systemd and nginx

- install-ubuntu-24.04-native.md (production) and -docker.md (alternative)

- deploy/systemd/sac-api.service, nginx/sac.conf.example, env.native.example

- deployment, README, INDEX, work-plan, TZ updated

deploy/.env.example

@@ -1,4 +1,5 @@
-# Скопировать: cp .env.example .env  (или в /etc/security-alert-center/.env)
+# Только для Docker Compose (см. docs/install-ubuntu-24.04-docker.md).
+# Production native: deploy/env.native.example → /etc/security-alert-center/sac-api.env
 
 # PostgreSQL
 POSTGRES_USER=sac