config(library-desk): add system settings database and scheduler integration

Add environment variables for:
- Central settings database (postgres-shared/system_settings)
- Scheduler service URL for task coordination

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

stacks/library-desk.yml

@@ -48,6 +48,16 @@ services:
       - REDIS_PORT=6379
       - REDIS_DB=4
 
+      # Central Settings Database
+      - SYSTEM_SETTINGS_HOST=postgres-shared
+      - SYSTEM_SETTINGS_PORT=5432
+      - SYSTEM_SETTINGS_DB=system_settings
+      - SYSTEM_SETTINGS_USER=settings
+      - SYSTEM_SETTINGS_PASSWORD=${SYSTEM_SETTINGS_PASSWORD}
+
+      # Scheduler Service
+      - SCHEDULER_URL=http://scheduler:8090
+
       # Python Configuration
       - PYTHONUNBUFFERED=1
       - TZ=${TZ:-Europe/Amsterdam}