Fix Mixed Content issue on staging (HTTPS/HTTP mismatch)

- Add Next.js API rewrites to proxy /api and /uploads requests
- Update docker-compose.staging.yml to use relative API paths
- Set NEXT_PUBLIC_API_URL to empty for staging (use rewrites)
- Add BACKEND_URL env var for Next.js server-side proxying
- Update all files to use relative paths when API_URL is empty

This fixes the issue where the staging site (HTTPS) was trying to
load resources from HTTP backend, causing Mixed Content errors.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/app/admin/cars/page.tsx

@@ -108,7 +108,7 @@ const FUEL_TYPES = [
   { value: 'LPG', label: 'LPG' },
 ];
 
-const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000';
+const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || '';
 
 // 이미지 URL 변환 (로컬 경로는 백엔드 URL 추가)
 const getImageUrl = (url: string | undefined): string => {

frontend/src/app/admin/dealers/page.tsx

@@ -5,7 +5,7 @@ import { useRouter } from 'next/navigation';
 import { useAuthStore } from '@/lib/store';
 import { useTranslation } from '@/lib/i18n';
 
-const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000';
+const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || '';
 
 interface DealerApplication {
   id: number;

frontend/src/app/admin/hero-banners/page.tsx

@@ -33,7 +33,7 @@ const defaultFormData: BannerFormData = {
   car_id: null,
 };
 
-const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000';
+const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || '';
 
 // 이미지 URL 변환 (로컬 경로는 백엔드 URL 추가)
 const getImageUrl = (url: string | undefined): string => {

frontend/src/app/admin/settings/page.tsx

@@ -42,7 +42,7 @@ interface ExchangeRateWeights {
   cny: number;
 }
 
-const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:8000';
+const API_BASE_URL = process.env.NEXT_PUBLIC_API_URL || '';
 
 export default function SettingsPage() {
   const [settings, setSettings] = useState<SystemSettings | null>(null);