feat(web): pass SYSTEM_ANNOUNCEMENT/SYSTEM_MAINTENANCE through docker-compose

Wire the two new web settings into the web service env block, matching the
NETWORK_ANNOUNCEMENT passthrough. SYSTEM_MAINTENANCE defaults to false.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker-compose.yml

@@ -332,6 +332,8 @@ services:
       - NETWORK_CONTACT_YOUTUBE=${NETWORK_CONTACT_YOUTUBE:-}
       - NETWORK_WELCOME_TEXT=${NETWORK_WELCOME_TEXT:-}
       - NETWORK_ANNOUNCEMENT=${NETWORK_ANNOUNCEMENT:-}
+      - SYSTEM_ANNOUNCEMENT=${SYSTEM_ANNOUNCEMENT:-}
+      - SYSTEM_MAINTENANCE=${SYSTEM_MAINTENANCE:-false}
       - CONTENT_HOME=/content
       - TZ=${TZ:-UTC}
       # Feature flags (set to false to disable specific pages)