feat(infra): HTTPS(TLS) 스캐폴딩 + 프록시 헤더 처리

- Spring prod: server.forward-headers-strategy=framework — nginx의 X-Forwarded-Proto를
  신뢰해 프록시 뒤에서도 request.isSecure()=true → Secure 쿠키·https 리다이렉트 정상화.
- frontend/nginx-tls.conf: 443 TLS 종료 + 80→443 리다이렉트(+ /api 프록시).
- infra/docker-compose.tls.yml: base와 함께 쓰는 HTTPS 오버레이(443 매핑·인증서 볼륨·
  ACS_PUBLIC_BASE_URL/ACS_COOKIE_SECURE). infra/certs/.gitignore로 인증서·키 커밋 제외.
- README: HTTPS 적용 절차(인증서 준비→.env→compose 오버레이) 갱신.

검증: forward-headers=framework + cookie.secure=true로 기동 후 X-Forwarded-Proto=https 유무 대조 —
헤더 있으면 XSRF-TOKEN·JSESSIONID 모두 Secure, 없으면 XSRF Secure 미부여(프록시 프로토콜 연동 확인).
자체서명 인증서 생성 확인. 전체 컨테이너 TLS e2e는 Docker 데몬 기동 시 별도 검증 필요.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/nginx-tls.conf

@@ -0,0 +1,43 @@
+# HTTPS variant of nginx.conf — used by the docker-compose.tls.yml overlay.
+# nginx terminates TLS and reverse-proxies /api to the backend over the internal network.
+# Mount a certificate at /etc/nginx/certs/{fullchain.pem,privkey.pem}.
+
+# Redirect all plain HTTP to HTTPS.
+server {
+    listen 80;
+    server_name _;
+    return 301 https://$host$request_uri;
+}
+
+server {
+    listen 443 ssl;
+    http2 on;
+    server_name _;
+
+    ssl_certificate     /etc/nginx/certs/fullchain.pem;
+    ssl_certificate_key /etc/nginx/certs/privkey.pem;
+    ssl_protocols       TLSv1.2 TLSv1.3;
+    ssl_ciphers         HIGH:!aNULL:!MD5;
+    ssl_prefer_server_ciphers on;
+
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # SPA client-side routing: fall back to index.html
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+
+    # Proxy API calls to the backend service (session cookie preserved).
+    # X-Forwarded-Proto=https lets the app (server.forward-headers-strategy=framework)
+    # know the original request was secure → Secure cookies + https redirects.
+    location /api/ {
+        proxy_pass http://app:8080;
+        proxy_http_version 1.1;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_cookie_path / /;
+    }
+}