feat: initialize backend with Express and CORS for password check API

- Add package.json with dependencies for express, cors, and dotenv
- Create server.js to handle password checking and health check endpoints
- Implement CORS with configurable allowed origins
- Ensure PASSWORD environment variable is set before starting the server

Dockerfile

@@ -0,0 +1,13 @@
+FROM node:22-alpine
+
+WORKDIR /app
+
+COPY package.json ./
+RUN npm install --omit=dev
+
+COPY server.js ./
+
+USER node
+EXPOSE 3001
+
+CMD ["node", "server.js"]