fix(infra): nginx 동적 DNS resolve + Docker 헬스체크 추가

컨테이너 재생성 시 502 Bad Gateway 방지:
- 모든 nginx proxy_pass를 set $upstream 변수 방식으로 전환 (9개 파일, 24개 location)
- resolver 127.0.0.11 valid=10s ipv6=off 통합 선언
- ai-api location의 개별 resolver 8.8.8.8 제거 (server-level로 통합)
- 10개 API 서비스에 healthcheck 추가 (Node: wget, Python: urllib)
- 모든 web/gateway depends_on을 condition: service_healthy로 강화

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

gateway/nginx.conf

@@ -1,6 +1,7 @@
 server {
     listen 80;
     server_name _;
+    resolver 127.0.0.11 valid=10s ipv6=off;
 
     root /usr/share/nginx/html;
 
@@ -33,7 +34,9 @@ server {
 
     # SSO Auth 프록시
     location /auth/ {
-        proxy_pass http://sso-auth:3000/api/auth/;
+        set $upstream http://sso-auth:3000;
+        rewrite ^/auth/(.*)$ /api/auth/$1 break;
+        proxy_pass $upstream;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
@@ -42,7 +45,8 @@ server {
 
     # System1 API 프록시 (대시보드 배너용: 알림/TBM/휴가 카운트)
     location /api/ {
-        proxy_pass http://system1-api:3005/api/;
+        set $upstream http://system1-api:3005;
+        proxy_pass $upstream$request_uri;
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;