#640 Platform Bootstrap Infrastructure

Create engine/platform/ directory structure and database migration. Directory structure: - engine/platform/__init__.py - engine/platform/constants.py (SYSTEM_TENANTS, INTERNAL_QUEUE) - engine/platform/bootstrap.py (bootstrap check and trigger) - engine/platform/config/ (YAML loaders) - engine/platform/tools/ (internal tools) - engine/platform/workflows/ (system workflows) Database migration: - ALTER tenant_clients: bootstrap_status, bootstrap_workflow_id, is_system_tenant, suspended_at, suspended_reason - CREATE highway.platform_status (singleton table for platform init state) Parent: #639