fix(monitor): refresh maintenance cache after loading sites

Move refreshMaintenanceCache() to after sites are loaded into
liveState. Previously it ran before the site loading loop, so on
the first iteration liveState was empty and the cache resolved
to nothing. This caused groups to briefly show DOWN on startup
for children that were in maintenance, until the next poll cycle
repopulated the cache.

internal/monitor/monitor.go

@@ -283,8 +283,6 @@ func (e *Engine) Start(ctx context.Context) {
 			default:
 			}
 
-			e.refreshMaintenanceCache(ctx)
-
 			configs, err := e.db.GetSites(ctx)
 			if err != nil {
 				e.AddLog(fmt.Sprintf("Failed to load sites: %v", err))
@@ -327,6 +325,10 @@ func (e *Engine) Start(ctx context.Context) {
 				}
 			}
 
+			// Refresh after sites load so the cache covers newly added sites.
+			// On first iteration liveState was empty before the loop above.
+			e.refreshMaintenanceCache(ctx)
+
 			e.mu.RLock()
 			var vanished []int
 			for id := range e.liveState {