stage-28: make the proxy docker network name configurable per environment

On AnD0R the reverse-proxy lives on the 'backend' docker network; on
cloud.neuronetz.ai it's 'neuronetz_default'. With a hardcoded name the
cockpit ended up on a network the prod proxy couldn't see and routing
silently dropped. Network is now overridable via PSYC_PROXY_NETWORK in
.env (default 'backend' keeps dev working).

On prod, set PSYC_PROXY_NETWORK=neuronetz_default in .env before the
next compose up.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docker-compose.yml

@@ -99,6 +99,10 @@ services:
               capabilities: [gpu]
 
 networks:
+  # The reverse-proxy + acme-companion need to share a docker network with the
+  # cockpit so they can see each other. The actual network name differs by
+  # environment (e.g. 'backend' in dev, 'neuronetz_default' in production), so
+  # it's overridable via PSYC_PROXY_NETWORK in .env. Default keeps dev working.
   backend:
-    name: backend
+    name: ${PSYC_PROXY_NETWORK:-backend}
     external: true