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.

docker/node/Dockerfile

@@ -0,0 +1,12 @@
+FROM node:20-alpine
+
+RUN npm install -g pm2
+
+WORKDIR /app
+
+COPY nodejs/package.json nodejs/package-lock.json ./
+RUN npm ci --omit=dev
+
+COPY nodejs/ ./
+
+CMD ["pm2-runtime", "ecosystem.config.js"]