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.

.env.example

@@ -2,7 +2,6 @@
 
 # --- Domains (Caddy) ---
 PUBLIC_DOMAIN=blog.localhost
-CMS_DOMAIN=cms.blog.localhost
 ACME_EMAIL=admin@example.com
 
 # --- Database ---
@@ -25,4 +24,5 @@ STRAPI_URL=http://cms:1337
 # Public base URL of the website, used for canonical URLs and Open Graph.
 PUBLIC_SITE_URL=https://blog.localhost
 # Public base URL of Strapi, used to build absolute media URLs in the browser.
-PUBLIC_STRAPI_URL=https://cms.blog.localhost
+# Same origin as PUBLIC_SITE_URL: Caddy proxies /admin and /uploads to Strapi.
+PUBLIC_STRAPI_URL=https://blog.localhost