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 => {