fix(audit): log system.upgrade trigger

Upgrade war die einzige system-Mutation ohne Audit-Eintrag. Alle
anderen (maintenance, service-restart, haproxy-reload, IPv6, render-
configs, auto-update) loggen bereits. Jetzt wird system.upgrade mit
actor + unit-Name geloggt.

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

internal/handlers/system.go

@@ -881,6 +881,10 @@ rm -f /var/lib/edgeguard/upgrade.sh
 		_ = fallback.Process.Release()
 	}
 
+	if h.Audit != nil {
+		_ = h.Audit.Log(c.Request.Context(), actorOf(c), "system.upgrade",
+			"", gin.H{"unit": unitName}, h.NodeID)
+	}
 	c.JSON(http.StatusAccepted, response.Envelope{
 		Data:    gin.H{"status": "upgrading", "unit": unitName},
 		Error:   nil,