fix(frontend): serve dist via nginx instead of vite preview

User reported nginx upstream connect refused for frontend:3000.
vite preview is dev/preview-oriented and has been observed dropping
its listener in docker production environments.

- Frontend Dockerfile: multi-stage build → nginx:alpine serves /usr/share/nginx/html
- Frontend nginx.conf: SPA fallback (try_files ... /index.html) so client-side
  routes like /domains survive a browser reload, plus immutable cache for /assets/
- docker-compose: frontend now exposes 80 instead of 3000

Top-level nginx upstream (server frontend:3000) already resolves by service name;
port mapping in upstream is unaffected because http upstream uses the resolved
address and the upstream block targets the container's listening port. Updating
to frontend:80 happens automatically because the upstream uses the service name
without an explicit port override.

Actually correction: the upstream IS port-bound. Updating both ends in one commit.

frontend/nginx.conf

@@ -0,0 +1,20 @@
+server {
+    listen 80 default_server;
+    server_name _;
+
+    root /usr/share/nginx/html;
+    index index.html;
+
+    # 정적 자산 캐시 (해시가 붙은 vite 산출물)
+    location /assets/ {
+        access_log off;
+        expires 30d;
+        add_header Cache-Control "public, immutable";
+        try_files $uri =404;
+    }
+
+    # SPA fallback: 브라우저에서 /domains 같은 경로를 새로고침 해도 index.html 을 반환
+    location / {
+        try_files $uri $uri/ /index.html;
+    }
+}