fix(docker): use unique Compose service name for API upstream

Rename stack service from backend to runners-calendar-backend so DNS on
shared external networks (e.g. postgres_default) cannot resolve to
another project’s backend. Nginx proxy_pass targets the same hostname.

Made-with: Cursor

docker/nginx.frontend.conf

@@ -10,7 +10,7 @@ server {
 
   # Браузер ходит на тот же origin: /api/* → бэкенд с тем же префиксом /api
   location /api/ {
-    proxy_pass http://backend:3000;
+    proxy_pass http://runners-calendar-backend:3000;
     proxy_http_version 1.1;
     proxy_set_header Host $host;
     proxy_set_header X-Real-IP $remote_addr;