refactor(models): split Site into SiteConfig + SiteState

Site now embeds SiteConfig (22 persistent fields) and SiteState
(11 ephemeral runtime fields). Field access unchanged via promotion
— site.Name and site.Status still work.

Store layer deals exclusively in SiteConfig — the DB never sees
runtime state. Engine's liveState keeps full Site composites.
UpdateSiteConfig reduced from 11-line field-by-field copy to
`existing.SiteConfig = cfg`.

RunCheck takes SiteConfig (only needs config fields). Checker is
now statically prevented from reading/writing runtime state.

Backup.Sites changed to []SiteConfig — exports no longer carry
zero-valued runtime fields. Import backward-compatible (json
ignores unknown fields).

internal/importer/kuma.go

@@ -3,9 +3,10 @@ package importer
 import (
 	"encoding/json"
 	"fmt"
-	"gitea.lerkolabs.com/lerkolabs/uptop/internal/models"
 	"os"
 	"strings"
+
+	"gitea.lerkolabs.com/lerkolabs/uptop/internal/models"
 )
 
 type KumaBackup struct {
@@ -80,7 +81,7 @@ func ConvertKuma(kb *KumaBackup) models.Backup {
 		}
 	}
 
-	var sites []models.Site
+	var sites []models.SiteConfig
 	for _, m := range kb.MonitorList {
 		site := convertKumaMonitor(m, kumaToUpkeepAlert)
 		sites = append(sites, site)
@@ -132,8 +133,8 @@ func convertKumaNotifications(entries []KumaNotifEntry) map[int]models.AlertConf
 	return result
 }
 
-func convertKumaMonitor(m KumaMonitor, alertMap map[int]int) models.Site {
-	site := models.Site{
+func convertKumaMonitor(m KumaMonitor, alertMap map[int]int) models.SiteConfig {
+	site := models.SiteConfig{
 		ID:          m.ID,
 		Name:        m.Name,
 		Description: m.Description,