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_26/v324.go

@@ -0,0 +1,24 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_26
+
+import (
+	"fmt"
+
+	"gitea.dev/modelmigration/base"
+)
+
+func FixClosedMilestoneCompleteness(x base.EngineMigration) error {
+	// Update all milestones to recalculate completeness with the new logic:
+	// - Closed milestones with 0 issues should show 100%
+	// - All other milestones should calculate based on closed/total ratio
+	_, err := x.Exec("UPDATE `milestone` SET completeness=(CASE WHEN is_closed = ? AND num_issues = 0 THEN 100 ELSE 100*num_closed_issues/(CASE WHEN num_issues > 0 THEN num_issues ELSE 1 END) END)",
+		true,
+	)
+	if err != nil {
+		return fmt.Errorf("error updating milestone completeness: %w", err)
+	}
+
+	return nil
+}