feat(18-05): add SoftwareVersion field to DeviceStatusEvent

- Additive field with omitempty tag for SNMP device identification
- Existing RouterOS events produce identical JSON (field not set)

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

poller/internal/bus/publisher.go

@@ -20,6 +20,7 @@ type DeviceStatusEvent struct {
 	DeviceID        string `json:"device_id"`
 	TenantID        string `json:"tenant_id"`
 	Status          string `json:"status"` // "online" or "offline"
+	SoftwareVersion string `json:"software_version,omitempty"` // parsed from sysDescr for SNMP devices
 	RouterOSVersion string `json:"routeros_version,omitempty"`
 	MajorVersion    int    `json:"major_version,omitempty"`
 	BoardName       string `json:"board_name,omitempty"`