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/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,