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/config/apply_test.go

@@ -114,8 +114,8 @@ func TestApplyUpdate(t *testing.T) {
 
 func TestApplyPrune(t *testing.T) {
 	s := newTestStore(t)
-	s.AddSite(context.Background(), models.Site{Name: "Keep", URL: "https://keep.com", Type: "http", Interval: 30, ExpiryThreshold: 7, Method: "GET", AcceptedCodes: "200-299"})
-	s.AddSite(context.Background(), models.Site{Name: "Remove", URL: "https://remove.com", Type: "http", Interval: 30, ExpiryThreshold: 7, Method: "GET", AcceptedCodes: "200-299"})
+	s.AddSite(context.Background(), models.SiteConfig{Name: "Keep", URL: "https://keep.com", Type: "http", Interval: 30, ExpiryThreshold: 7, Method: "GET", AcceptedCodes: "200-299"})
+	s.AddSite(context.Background(), models.SiteConfig{Name: "Remove", URL: "https://remove.com", Type: "http", Interval: 30, ExpiryThreshold: 7, Method: "GET", AcceptedCodes: "200-299"})
 
 	f := &File{
 		Monitors: []Monitor{
@@ -191,7 +191,7 @@ func TestApplyGroupHierarchy(t *testing.T) {
 	}
 
 	sites, _ := s.GetSites(context.Background())
-	var group models.Site
+	var group models.SiteConfig
 	for _, s := range sites {
 		if s.Type == "group" {
 			group = s