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_27/v339.go

@@ -0,0 +1,40 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_27
+
+import (
+	"context"
+
+	"gitea.dev/modelmigration/base"
+
+	"xorm.io/xorm/schemas"
+)
+
+// AddCreatedUnixToActionUserIsDeletedIndex extends the c_u composite index on
+// the action table to include created_unix, enabling efficient ORDER BY on the
+// dashboard feed query without a full sort of all matching rows.
+func AddCreatedUnixToActionUserIsDeletedIndex(x base.EngineMigration) error {
+	// xorm Sync cannot reliably update an index when another index already
+	// covers the same columns in a different order (Equal() is order-insensitive).
+	// Drop the old c_u index explicitly, then recreate it with the new column set.
+	indexes, err := x.Dialect().GetIndexes(x.DB(), context.Background(), "action")
+	if err != nil {
+		return err
+	}
+	for _, idx := range indexes {
+		if idx.Name == "c_u" {
+			if _, err := x.Exec(x.Dialect().DropIndexSQL("action", idx)); err != nil {
+				return err
+			}
+			break
+		}
+	}
+
+	newIndex := schemas.NewIndex("c_u", schemas.IndexType)
+	newIndex.AddColumn("user_id", "is_deleted", "created_unix")
+	if _, err := x.Exec(x.Dialect().CreateIndexSQL("action", newIndex)); err != nil {
+		return err
+	}
+	return nil
+}