refactor: prepare to decouple the "model migration" package and "models" package (#38533)

Migrations should never use model structs directly, because the model
structs can be different in different releases. e.g. if one migration uses
"User" model, it works in the early releases, then one day, when the
User model changes, the migration breaks because it will use the
new (incorrect) User model, it should only use the old User model.

The same to "modules/structs".

---------

Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: delvh <dev.lh@web.de>

modelmigration/v1_23/v303.go

@@ -0,0 +1,29 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_23
+
+import (
+	"gitea.dev/modelmigration/base"
+
+	"xorm.io/xorm"
+)
+
+// CommentMetaData stores metadata for a comment, these data will not be changed once inserted into database
+type CommentMetaData struct {
+	ProjectColumnID    int64  `json:"project_column_id"`
+	ProjectColumnTitle string `json:"project_column_title"`
+	ProjectTitle       string `json:"project_title"`
+}
+
+func AddCommentMetaDataColumn(x base.EngineMigration) error {
+	type Comment struct {
+		CommentMetaData *CommentMetaData `xorm:"JSON TEXT"` // put all non-index metadata in a single field
+	}
+
+	_, err := x.SyncWithOptions(xorm.SyncOptions{
+		IgnoreConstrains: true,
+		IgnoreIndices:    true,
+	}, new(Comment))
+	return err
+}