Add bare /health location to nginx for management monitor compatibility

The management monitor checks http://{host}:{port}/health (no slug prefix).
Adds a location = /health alias alongside the existing /rates/health
so both the slug URL (browser/NPM) and the bare path (monitor) work.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/nginx.conf

@@ -40,11 +40,15 @@ server {
     client_max_body_size 10M;
   }
 
-  # 3. Health
+  # 3. Health (slugged for browser use + bare for management monitor)
   location /rates/health {
     proxy_pass http://backend:8000/health;
   }
 
+  location = /health {
+    proxy_pass http://backend:8000/health;
+  }
+
   # 4. SPA fallback
   location /rates/ {
     root /usr/share/nginx/html;