MM-45956: Optimize FileInfo stats query (#22603)

* MM-45956: Optimize FileInfo stats query

We Denormalize Post.ChannelId on FileInfo.ChannelId

```release-note
The file info stats query is now optimized by denormalizing the channelID column into the table itself. This will speed up the query to get the file count for a channel on clicking the RHS.

Migration times:
On a MySQL 8.0.31 DB with
1405 rows in FileInfo and 11M posts, it took around 0.3s

On a Postgres 12.14 DB with
1731 rows in FileInfo and 11M posts, it took around 0.27s
```

https://mattermost.atlassian.net/browse/MM-45956

server/channels/app/file.go

@@ -274,6 +274,7 @@ func (a *App) getInfoForFilename(post *model.Post, teamID, channelID, userID, ol
 	info.Id = model.NewId()
 	info.CreatorId = post.UserId
 	info.PostId = post.Id
+	info.ChannelId = post.ChannelId
 	info.CreateAt = post.CreateAt
 	info.UpdateAt = post.UpdateAt
 	info.Path = path
@@ -1214,6 +1215,7 @@ func (a *App) CopyFileInfos(userID string, fileIDs []string) ([]string, *model.A
 		fileInfo.CreateAt = now
 		fileInfo.UpdateAt = now
 		fileInfo.PostId = ""
+		fileInfo.ChannelId = ""
 
 		if _, err := a.Srv().Store().FileInfo().Save(fileInfo); err != nil {
 			var appErr *model.AppError

server/channels/app/file_test.go

@@ -368,6 +368,7 @@ func TestSearchFilesInTeamForUser(t *testing.T) {
 			fileInfo, err := th.App.Srv().Store().FileInfo().Save(&model.FileInfo{
 				CreatorId: th.BasicUser.Id,
 				PostId:    th.BasicPost.Id,
+				ChannelId: th.BasicPost.ChannelId,
 				Name:      searchTerm,
 				Path:      searchTerm,
 				Extension: "jpg",

server/channels/app/import_functions.go

@@ -1638,7 +1638,7 @@ func (a *App) uploadAttachments(c request.CTX, attachments *[]imports.Attachment
 
 func (a *App) updateFileInfoWithPostId(post *model.Post) {
 	for _, fileID := range post.FileIds {
-		if err := a.Srv().Store().FileInfo().AttachToPost(fileID, post.Id, post.UserId); err != nil {
+		if err := a.Srv().Store().FileInfo().AttachToPost(fileID, post.Id, post.ChannelId, post.UserId); err != nil {
 			mlog.Error("Error attaching files to post.", mlog.String("post_id", post.Id), mlog.Any("post_file_ids", post.FileIds), mlog.Err(err))
 		}
 	}

server/channels/app/post.go

@@ -416,7 +416,7 @@ func (a *App) addPostPreviewProp(post *model.Post) (*model.Post, error) {
 func (a *App) attachFilesToPost(post *model.Post) *model.AppError {
 	var attachedIds []string
 	for _, fileID := range post.FileIds {
-		err := a.Srv().Store().FileInfo().AttachToPost(fileID, post.Id, post.UserId)
+		err := a.Srv().Store().FileInfo().AttachToPost(fileID, post.Id, post.ChannelId, post.UserId)
 		if err != nil {
 			mlog.Warn("Failed to attach file to post", mlog.String("file_id", fileID), mlog.String("post_id", post.Id), mlog.Err(err))
 			continue