MM-58038: Use context to call master for DeletePost (#27098)

Calling app.DeletePost immediately after creating a post
is susceptible to replica lag because we were calling the
replica to check for the post.

We fix this by passing a context to always query master.

https://mattermost.atlassian.net/browse/MM-58038

```release-note
NONE
```

server/channels/store/timerlayer/timerlayer.go

@@ -6122,10 +6122,10 @@ func (s *TimerLayerPostStore) GetRepliesForExport(parentID string) ([]*model.Rep
 	return result, err
 }
 
-func (s *TimerLayerPostStore) GetSingle(id string, inclDeleted bool) (*model.Post, error) {
+func (s *TimerLayerPostStore) GetSingle(rctx request.CTX, id string, inclDeleted bool) (*model.Post, error) {
 	start := time.Now()
 
-	result, err := s.PostStore.GetSingle(id, inclDeleted)
+	result, err := s.PostStore.GetSingle(rctx, id, inclDeleted)
 
 	elapsed := float64(time.Since(start)) / float64(time.Second)
 	if s.Root.Metrics != nil {