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/api4/channel.go

@@ -1750,7 +1750,7 @@ func addChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 	audit.AddEventParameter(auditRec, "post_root_id", postRootId)
 
 	if ok && len(postRootId) == 26 {
-		rootPost, err := c.App.GetSinglePost(postRootId, false)
+		rootPost, err := c.App.GetSinglePost(c.AppContext, postRootId, false)
 		if err != nil {
 			c.Err = err
 			return