fix: update nginx for Astro SSG — /_astro/ cache, remove SPA fallback

infra/docker/nginx/nginx-frontend.conf

@@ -1,4 +1,4 @@
-# Nginx config for serving Vite-built static frontend files
+# Nginx config for serving Astro-built static frontend files
 server {
     listen 3000;
     server_name _;
@@ -11,14 +11,15 @@ server {
     gzip_types text/plain text/css application/json application/javascript image/svg+xml;
     gzip_min_length 256;
 
-    # Cache static assets (JS/CSS hashed filenames)
-    location /assets/ {
+    # Cache Astro hashed assets (/_astro/) — immutable since hash changes
+    location /_astro/ {
         expires 1y;
         add_header Cache-Control "public, immutable";
     }
 
-    # SPA fallback — all non-file routes serve index.html
+    # Astro SSG pages — each route is a real directory with index.html
+    # Fallback to 404 instead of index.html (no SPA catch-all)
     location / {
-        try_files $uri $uri/ /index.html;
+        try_files $uri $uri/ =404;
     }
 }