Replace S3 storage with Docker-mounted filesystem

- Add Docker and Compose deployment with persistent HTML and Postgres volumes
- Remove AWS storage configuration and dependency
- Document local Docker setup and filesystem storage

Dockerfile

@@ -0,0 +1,20 @@
+FROM node:20-alpine
+
+WORKDIR /app
+
+COPY package.json ./
+RUN npm install --omit=dev && npm cache clean --force
+
+COPY src ./src
+
+ENV NODE_ENV=production
+ENV POSTPLAN_HTML_DIR=/data/html
+
+RUN mkdir -p /data/html
+
+EXPOSE 3000
+
+HEALTHCHECK --interval=10s --timeout=5s --start-period=10s --retries=10 \
+  CMD node -e "fetch('http://127.0.0.1:3000/healthz').then((r)=>process.exit(r.ok?0:1)).catch(()=>process.exit(1))"
+
+CMD ["node", "src/server.js"]