fix: Update npm to latest version and fix Tailwind v4 PostCSS configuration

- Install @tailwindcss/postcss for Tailwind v4 compatibility
- Update postcss.config.js to use new Tailwind PostCSS plugin
- Update npm to latest version in backend Dockerfile
- Fixes build failures with Next.js 16.1.6 and Tailwind CSS v4.2.0

backend/Dockerfile

@@ -1,8 +1,8 @@
 FROM node:20-alpine AS base
 WORKDIR /app
 
-# Install Bun
-RUN npm install -g bun
+# Install Bun and update npm
+RUN npm install -g bun && npm install -g npm@latest
 
 # Install dependencies
 COPY package.json bun.lockb* ./