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.

caddy/Caddyfile

@@ -15,15 +15,21 @@
 {$PUBLIC_DOMAIN} {
 	import security_headers
 	encode zstd gzip
-	reverse_proxy frontend:3000
-}
 
-{$CMS_DOMAIN} {
-	import security_headers
-	encode zstd gzip
-	# Uploaded media can be large; Strapi's own limit still applies.
-	request_body {
-		max_size 100MB
+	# /admin and /uploads go to Strapi; everything else to the frontend.
+	# Strapi 5 nests the whole admin panel (UI + its own API) under /admin,
+	# so this single prefix is enough - it never touches /api, which stays
+	# reserved for the frontend's own Nitro endpoints.
+	@cms path /admin* /uploads*
+	handle @cms {
+		# Uploaded media can be large; Strapi's own limit still applies.
+		request_body {
+			max_size 100MB
+		}
+		reverse_proxy cms:1337
+	}
+
+	handle {
+		reverse_proxy frontend:3000
 	}
-	reverse_proxy cms:1337
 }