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/cluster/cluster_test.go

@@ -203,7 +203,7 @@ func TestProbeRegister_Failure(t *testing.T) {
 func TestProbeFetchAssignments_Success(t *testing.T) {
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		json.NewEncoder(w).Encode(map[string][]models.Site{
-			"sites": {{ID: 1, Name: "s1", Type: "http", URL: "http://example.com"}},
+			"sites": {{SiteConfig: models.SiteConfig{ID: 1, Name: "s1", Type: "http", URL: "http://example.com"}}},
 		})
 	}))
 	defer srv.Close()
@@ -240,8 +240,8 @@ func TestProbeExecuteChecks(t *testing.T) {
 	defer srv.Close()
 
 	sites := []models.Site{
-		{ID: 1, Type: "http", URL: srv.URL},
-		{ID: 2, Type: "http", URL: srv.URL},
+		{SiteConfig: models.SiteConfig{ID: 1, Type: "http", URL: srv.URL}},
+		{SiteConfig: models.SiteConfig{ID: 2, Type: "http", URL: srv.URL}},
 	}
 
 	strict := &http.Client{}
@@ -277,7 +277,7 @@ func TestProbeExecuteChecks_Concurrency(t *testing.T) {
 
 	var sites []models.Site
 	for i := 0; i < 20; i++ {
-		sites = append(sites, models.Site{ID: i + 1, Type: "http", URL: srv.URL})
+		sites = append(sites, models.Site{SiteConfig: models.SiteConfig{ID: i + 1, Type: "http", URL: srv.URL}})
 	}
 
 	results := probeExecuteChecks(context.Background(), sites, &http.Client{}, &http.Client{}, true)

internal/cluster/probe.go

@@ -152,7 +152,7 @@ loop:
 			defer wg.Done()
 			defer func() { <-sem }()
 
-			cr := monitor.RunCheck(ctx, s, strict, insecure, false, allowPrivate)
+			cr := monitor.RunCheck(ctx, s.SiteConfig, strict, insecure, false, allowPrivate)
 			mu.Lock()
 			results = append(results, probeResultItem{
 				SiteID:      s.ID,