chore(flags): wire UNLEASH_URL and the SDK cache volume into the stacks

Both variables default to empty, so an environment without Unleash has
every flag off — the correct dark state rather than a boot failure.

The cache volume is the mitigation for the one real regression risk in
this design: the SDK evaluates everything False until it syncs, so a
backend cold-starting with an empty cache while Unleash is unreachable
would make a *released* feature disappear. On a named volume the disk
cache survives a restart.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

docker-compose.portainer.yml

@@ -7,6 +7,8 @@
 #   ADMIN_API_KEY          — Backend admin API key
 #   TYPESENSE_API_KEY      — Typesense admin API key
 #   TYPESENSE_SEARCH_KEY   — Typesense search-only key (exposed to frontend)
+#   UNLEASH_URL            — http://<unleash-ip>:4242/api (empty = all flags off)
+#   UNLEASH_API_TOKEN      — Unleash *client* token, environment: production
 #   AIRFLOW_ADMIN_USER     — Airflow admin username (password auto-generated, see api-server logs)
 
 services:
@@ -44,6 +46,12 @@ services:
       ADMIN_API_KEY: ${ADMIN_API_KEY:-changeme}
       TYPESENSE_URL: http://typesense:8108
       TYPESENSE_API_KEY: ${TYPESENSE_API_KEY:-changeme}
+      # Unset means every feature flag is False — the correct dark state for an
+      # environment with no Unleash, not a failure.
+      UNLEASH_URL: ${UNLEASH_URL:-}
+      UNLEASH_API_TOKEN: ${UNLEASH_API_TOKEN:-}
+    volumes:
+      - unleash_cache:/app/.unleash
     depends_on:
       sc_database:
         condition: service_healthy
@@ -201,3 +209,4 @@ volumes:
   postgres_data:
   typesense_data:
   airflow_logs:
+  unleash_cache: