#647 Platform API Endpoints

REST API endpoints for platform tenant management. Endpoints (require platform_admin role): - POST /api/v1/platform/tenants - Create tenant (triggers onboarding workflow) - GET /api/v1/platform/tenants - List all tenants with status - GET /api/v1/platform/tenants/{id} - Get tenant details - POST /api/v1/platform/tenants/{id}/suspend - Suspend tenant - POST /api/v1/platform/tenants/{id}/reactivate - Reactivate tenant - GET /api/v1/platform/status - Platform initialization status Blueprint: api/blueprints/v1/platform.py Parent: #639