Externalise WEBUI_URL / LLM_URL to .env

So changing the deployment's hostnames is a one-file edit (.env) instead
of touching docker-compose.yml. WEBUI_URL is the full URL with scheme
(Open WebUI uses it for auth redirects); LLM_URL is the bare hostname
(Anubis wants it for COOKIE_DOMAIN).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

deployments/ai-stack/.env.example

@@ -1,6 +1,14 @@
 # Copy to .env and fill in. docker compose picks .env up automatically.
 # Never commit the populated .env — it's in .gitignore at repo root.
 
+# Public URL Open WebUI is reached at, with scheme. Used for auth redirects
+# and email-link generation.
+WEBUI_URL=https://chat.example.com
+
+# Bare hostname (no scheme) for the same Open WebUI host. Used as the cookie
+# domain by the Anubis sidecar so its session cookie scopes correctly.
+LLM_URL=chat.example.com
+
 # Cookie-signing key for Open WebUI sessions. Generate once with:
 #   openssl rand -hex 32
 # Must stay stable across restarts — rotating it logs every user out.