docs(deploy): add install guide for a domain with HTTPS

- docs/install-guide.md: step-by-step server setup (DNS, Docker, install.sh,
  .env, launch, checks), web and Android client setup, own reverse proxy
  variant, updates, backups and troubleshooting
- deploy/compose.tls.yaml + Caddyfile: optional Caddy with automatic
  Let's Encrypt in front of nginx (enabled via COMPOSE_FILE in .env)
- nginx: honour X-Forwarded-Proto/For from private networks
- .env.example: DOMAIN, ACME_EMAIL, COMPOSE_FILE, bootstrap admin vars
- install.sh: create caddy data dirs; release.yml: ship TLS files as assets
- deployment.md, docs index and README link to the guide

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -6,6 +6,7 @@ Self-hosted музыкальный сервис: веб и Android-прилож
 - Идея продукта: [docs/IDEA.md](docs/IDEA.md)
 - Документация для разработки: [docs/README.md](docs/README.md)
 - План: [docs/roadmap.md](docs/roadmap.md)
+- Установка сервера на свой домен: [docs/install-guide.md](docs/install-guide.md)
 
 ## Структура