Serve the CMS admin panel under /admin on the public domain

Replace the separate CMS subdomain with path-based routing in Caddy:
/admin and /uploads go to Strapi, everything else to the frontend.
Strapi 5 nests its whole admin panel (UI and API) under /admin, so this
one prefix is enough and never collides with the frontend's own /api
routes. Drops CMS_DOMAIN and the unused STRAPI_URL env var on the cms
service; PUBLIC_STRAPI_URL now points at the same origin as the site.

Authentication is unchanged: Strapi's own admin login still gates the
panel, this only changes how it's reached.

docker-compose.yml

@@ -36,7 +36,6 @@ services:
       TRANSFER_TOKEN_SALT: ${TRANSFER_TOKEN_SALT}
       JWT_SECRET: ${JWT_SECRET}
       ENCRYPTION_KEY: ${ENCRYPTION_KEY}
-      STRAPI_URL: https://${CMS_DOMAIN}
     volumes:
       - cms-uploads:/app/public/uploads
 
@@ -71,7 +70,6 @@ services:
       - 8.8.8.8
     environment:
       PUBLIC_DOMAIN: ${PUBLIC_DOMAIN}
-      CMS_DOMAIN: ${CMS_DOMAIN}
       ACME_EMAIL: ${ACME_EMAIL}
     volumes:
       - ./caddy/Caddyfile:/etc/caddy/Caddyfile:ro