Add Docker Compose production stack (php-apache, mariadb, node/pm2)

Single-command deploy: docker compose up -d --build brings up the LEMP
stack plus the Node realtime/scheduler service. app/config.php and DB
secrets are generated from .env at container start, never baked into
the image or committed.

.env.example

@@ -0,0 +1,17 @@
+# Copy to .env and fill in real values before running: docker compose up -d --build
+
+# MariaDB root password (also used by the app's db_pass)
+DB_ROOT_PASSWORD=
+
+# Public IP or domain the browser uses to reach this server (Socket.IO client URL)
+PUBLIC_HOST=
+
+# Shared secret between PHP and Node (must be a long random string)
+EMIT_SECRET=
+
+# SMTP (Gmail) used for outgoing mail
+SMTP_USERNAME=
+SMTP_PASSWORD=
+
+# Port to expose the web app on (default 80)
+HTTP_PORT=80