feat: agent version tracking, update button, device test, COM port list

- Migration 043: agent_version in workstations, netup device type
- agent-ws: sendCommandAndWait for async commands, save version on hello
- workstations route: list-ports, test-device, update, update-all endpoints
- agent-release route: GET /api/agents/latest-release reads latest.yml
- app.ts: serve /agent-updates/ static files
- api.ts: fix camelCase→snake_case for outgoing requests (fixes null:null)
- api.ts: new methods testDevice, sendUpdate, updateAll, listPorts
- EquipmentPage: version badge, update button, test (zap) button per device
- EquipmentPage: COM-port dropdown loaded from agent when online

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/migrations/043_agent_version.sql

@@ -0,0 +1,5 @@
+-- Track agent version per workstation
+ALTER TABLE workstations ADD COLUMN IF NOT EXISTS agent_version VARCHAR(20);
+-- Extend device types to include netup
+ALTER TABLE workstation_devices DROP CONSTRAINT IF EXISTS workstation_devices_type_check;
+ALTER TABLE workstation_devices ADD CONSTRAINT workstation_devices_type_check CHECK (type IN ('kkt', 'printer', 'netup'));