fix(engine): apply convergence + push/group check history

1. Poll loop now fully converges with the DB: updated site configs
   are refreshed via UpdateSiteConfig, and sites removed from the DB
   are evicted from liveState. Previously the loop only added new
   sites — config edits via apply were ignored until restart, and
   pruned sites kept being checked and alerting.

2. Push monitors now record check history on each heartbeat via
   recordCheck. Previously RecordHeartbeat updated state but never
   wrote to check_history — push uptime % and sparklines were empty.

3. Groups record a synthetic check per evaluation tick so they get
   uptime history and sparklines instead of blank displays.

internal/monitor/monitor.go

@@ -375,6 +375,8 @@ func (e *Engine) RecordHeartbeat(token string) bool {
 		go e.triggerAlert(alertID, "✅ RECOVERY", fmt.Sprintf("Push Monitor '%s' is receiving heartbeats.%s", name, downDur))
 	}
 
+	e.recordCheck(targetID, 0, true)
+
 	if prevStatus != models.StatusUp && prevStatus != models.StatusPending {
 		e.enqueueWrite(writeStateChange{siteID: targetID, fromStatus: string(prevStatus), toStatus: string(models.StatusUp)})
 	}
@@ -428,9 +430,11 @@ func (e *Engine) Start(ctx context.Context) {
 				}
 				continue
 			}
+			dbIDs := make(map[int]bool, len(configs))
 			for _, cfg := range configs {
+				dbIDs[cfg.ID] = true
 				e.mu.RLock()
-				_, exists := e.liveState[cfg.ID]
+				existing, exists := e.liveState[cfg.ID]
 				e.mu.RUnlock()
 				if !exists {
 					e.mu.Lock()
@@ -453,9 +457,24 @@ func (e *Engine) Start(ctx context.Context) {
 						defer e.checkerWG.Done()
 						e.monitorRoutine(ctx, id)
 					}(cfg.ID)
+				} else if existing.SiteConfig != cfg {
+					e.UpdateSiteConfig(cfg)
 				}
 			}
 
+			e.mu.RLock()
+			var vanished []int
+			for id := range e.liveState {
+				if !dbIDs[id] {
+					vanished = append(vanished, id)
+				}
+			}
+			e.mu.RUnlock()
+			for _, id := range vanished {
+				e.RemoveSite(id)
+				e.AddLog(fmt.Sprintf("Monitor removed (no longer in DB): ID %d", id))
+			}
+
 			select {
 			case <-time.After(pollInterval):
 			case <-ctx.Done():
@@ -1017,6 +1036,7 @@ func (e *Engine) checkGroup(_ context.Context, site models.Site) {
 	e.applyState(site.ID, func(s *models.Site) {
 		s.Status = status
 	})
+	e.recordCheck(site.ID, 0, !status.IsBroken())
 }
 
 func (e *Engine) EnqueueProbeCheck(siteID int, nodeID string, latencyNs int64, isUp bool) {