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/post.go

@@ -563,7 +563,7 @@ func getEditHistoryForPost(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	originalPost, err := c.App.GetSinglePost(c.Params.PostId, false)
+	originalPost, err := c.App.GetSinglePost(c.AppContext, c.Params.PostId, false)
 	if err != nil {
 		c.SetPermissionError(model.PermissionEditPost)
 		return
@@ -600,7 +600,7 @@ func deletePost(c *Context, w http.ResponseWriter, _ *http.Request) {
 	defer c.LogAuditRecWithLevel(auditRec, app.LevelContent)
 	audit.AddEventParameter(auditRec, "post_id", c.Params.PostId)
 
-	post, err := c.App.GetSinglePost(c.Params.PostId, false)
+	post, err := c.App.GetSinglePost(c.AppContext, c.Params.PostId, false)
 	if err != nil {
 		c.SetPermissionError(model.PermissionDeletePost)
 		return
@@ -844,7 +844,7 @@ func updatePost(c *Context, w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	originalPost, err := c.App.GetSinglePost(c.Params.PostId, false)
+	originalPost, err := c.App.GetSinglePost(c.AppContext, c.Params.PostId, false)
 	if err != nil {
 		c.SetPermissionError(model.PermissionEditPost)
 		return
@@ -916,7 +916,7 @@ func patchPost(c *Context, w http.ResponseWriter, r *http.Request) {
 	// Updating the file_ids of a post is not a supported operation and will be ignored
 	post.FileIds = nil
 
-	originalPost, err := c.App.GetSinglePost(c.Params.PostId, false)
+	originalPost, err := c.App.GetSinglePost(c.AppContext, c.Params.PostId, false)
 	if err != nil {
 		c.SetPermissionError(model.PermissionEditPost)
 		return
@@ -1023,7 +1023,7 @@ func saveIsPinnedPost(c *Context, w http.ResponseWriter, isPinned bool) {
 	audit.AddEventParameter(auditRec, "post_id", c.Params.PostId)
 	defer c.LogAuditRecWithLevel(auditRec, app.LevelContent)
 
-	post, err := c.App.GetSinglePost(c.Params.PostId, false)
+	post, err := c.App.GetSinglePost(c.AppContext, c.Params.PostId, false)
 	if err != nil {
 		c.SetPermissionError(model.PermissionReadChannelContent)
 		return
@@ -1117,7 +1117,7 @@ func unacknowledgePost(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	_, err := c.App.GetSinglePost(c.Params.PostId, false)
+	_, err := c.App.GetSinglePost(c.AppContext, c.Params.PostId, false)
 	if err != nil {
 		c.Err = err
 		return