Move issue pin to an standalone table for querying performance (#33452)

Noticed a SQL in gitea.com has a bigger load. It seems both `is_pull`
and `pin_order` are not indexed columns in the database.

```SQL
SELECT `id`, `repo_id`, `index`, `poster_id`, `original_author`, `original_author_id`, `name`, `content`, `content_version`, `milestone_id`, `priority`, `is_closed`, `is_pull`, `num_comments`, `ref`, `pin_order`, `deadline_unix`, `created_unix`, `updated_unix`, `closed_unix`, `is_locked`, `time_estimate` FROM `issue` WHERE (repo_id =?) AND (is_pull = 0) AND (pin_order > 0) ORDER BY pin_order
```

I came across a comment
https://github.com/go-gitea/gitea/pull/24406#issuecomment-1527747296
from @delvh , which presents a more reasonable approach. Based on this,
this PR will migrate all issue and pull request pin data from the
`issue` table to the `issue_pin` table. This change benefits larger
Gitea instances by improving scalability and performance.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/migrations/migrations.go

@@ -373,6 +373,7 @@ func prepareMigrationTasks() []*migration {
 
 		// Gitea 1.23.0-rc0 ends at migration ID number 311 (database version 312)
 		newMigration(312, "Add DeleteBranchAfterMerge to AutoMerge", v1_24.AddDeleteBranchAfterMergeForAutoMerge),
+		newMigration(313, "Move PinOrder from issue table to a new table issue_pin", v1_24.MovePinOrderToTableIssuePin),
 	}
 	return preparedMigrations
 }

models/migrations/v1_24/v313.go

@@ -0,0 +1,31 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_24 //nolint
+
+import (
+	"code.gitea.io/gitea/models/migrations/base"
+
+	"xorm.io/xorm"
+)
+
+func MovePinOrderToTableIssuePin(x *xorm.Engine) error {
+	type IssuePin struct {
+		ID       int64 `xorm:"pk autoincr"`
+		RepoID   int64 `xorm:"UNIQUE(s) NOT NULL"`
+		IssueID  int64 `xorm:"UNIQUE(s) NOT NULL"`
+		IsPull   bool  `xorm:"NOT NULL"`
+		PinOrder int   `xorm:"DEFAULT 0"`
+	}
+
+	if err := x.Sync(new(IssuePin)); err != nil {
+		return err
+	}
+
+	if _, err := x.Exec("INSERT INTO issue_pin (repo_id, issue_id, is_pull, pin_order) SELECT repo_id, id, is_pull, pin_order FROM issue WHERE pin_order > 0"); err != nil {
+		return err
+	}
+	sess := x.NewSession()
+	defer sess.Close()
+	return base.DropTableColumns(sess, "issue", "pin_order")
+}