#134 Implement Worker Registration and Capacity Management System

Implement enterprise-grade worker registration where workers: 1. Ping DB every 30 seconds announcing availability 2. Check allowed batch_size from DB (0 = paused, don't pick up work) 3. Enable admins to control capacity per worker/machine 4. Track worker presence (know which workers are online) 5. Support future brain component for automated capacity allocation IMPLEMENTATION: - Phase 1 (Now): Core registration with worker_registry table, heartbeat, capacity control - Phases 2-4 (Future): Worker groups, affinity rules, brain component FILES TO CREATE/MODIFY: - engine/migrations/sql/highway_0.0.47_worker_registration.sql (NEW) - engine/services/worker_registration_service.py (NEW) - engine/cli/worker.py (modify) - api/blueprints/v1/workers.py (NEW) - api/blueprints/v1/__init__.py (modify) KEY DESIGN: - 30s heartbeat interval, 90s timeout (3 missed = offline) - UPSERT-based idempotent registration - Graceful degradation: if DB unavailable, use CLI default batch_size - Status states: online, offline, paused, draining - Backward compatible: workers without registration still work