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>

deploy/Caddyfile

@@ -0,0 +1,18 @@
+# Caddy: автоматический HTTPS для Pipu Music. Переменные берутся из .env через compose.tls.yaml.
+{
+	email {$ACME_EMAIL}
+}
+
+{$DOMAIN} {
+	encode zstd gzip
+
+	# Загрузки в очередь на добавление идут чанками по 8 МБ (tus)
+	request_body {
+		max_size 16MB
+	}
+
+	reverse_proxy nginx:80 {
+		# не буферизовать стриминг аудио
+		flush_interval -1
+	}
+}