Add prod compose and backend production image.

Wire postgres, pgadmin, backend, nginx, and tor with nullcart-style layout; document clearnet and onion hostname env vars for deploy.

backend/Dockerfile.prod

@@ -0,0 +1,15 @@
+FROM node:22.23.4-alpine
+
+WORKDIR /backend
+
+RUN apk add --no-cache curl
+
+COPY package*.json .
+
+RUN npm ci
+
+COPY . .
+
+RUN npm run build
+
+CMD ["node", "dist/main"]