Add application registration API (POST /api/apps/register) that auto-assigns
ports. Portal now tracks app status (已注册/已上线/已离线) via scheduled health
checks every 30s on /actuator/health. Dashboard only shows ONLINE+enabled apps.
Admin UI displays status with colored indicators. Updated all documentation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>