fix: Bind Next.js server to 0.0.0.0 for external access

The deployment was unreachable because the Next.js server was binding
to localhost:3000 (127.0.0.1) instead of 0.0.0.0, making it only
accessible inside the Docker container.

- Added HOSTNAME=0.0.0.0 to Dockerfile build and runtime stages
- Added HOSTNAME=0.0.0.0 to docker-compose.yml for nextjs service

This allows the server to accept connections from external hosts.

frontend/Dockerfile

@@ -13,6 +13,9 @@ RUN npm install -g bun
 WORKDIR /app
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
+# Set environment for build to ensure server binds to all interfaces
+ENV HOSTNAME="0.0.0.0"
+ENV PORT="3000"
 RUN bun run build
 
 # Stage 3: Runtime
@@ -36,5 +39,7 @@ EXPOSE 3000
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
   CMD node -e "require('http').get('http://localhost:3000', (r) => {if (r.statusCode !== 200) throw new Error(r.statusCode)})"
 
-# Start standalone server
+# Start standalone server - bind to all interfaces (0.0.0.0) for external access
+ENV HOSTNAME="0.0.0.0"
+ENV PORT="3000"
 CMD ["node", "server.js"]