[v12.0/forgejo] fix: corrupted wiki unit default permission (#8234 follow-up) (#8439)

**Backport:** https://codeberg.org/forgejo/forgejo/pulls/8258

Closes #8119, follow-up of #8234

#8234 "only" fixed the case when a new wiki setting was applied.

However it lacked 2 aspects:
- fixing the already corrupted unit permission in the database (required a migration)
- fixing the API route

Both aspects should now be covered.

Additionally, I commented out the unused `UnitAccessMode` and indicated that they are only used for the wiki-unit (hopefully saving some time to future code-readers).

### Testing

- go to a commit before #8234 (e.g. 285f66b782)
- create 3 repositories
  - save the wiki settings of the second, without any change
  - set the wiki of the third to be globally writable
- verify the `default_permissions` column in the database, of the unit `5`: 0, 2, 3
- stop Forgejo, switch to this PR and start Forgejo
- verify that the logs writes `Migration[35]: Fix wiki unit default permission`
- verify the `default_permissions` column in the database, of the unit `5`: 0, 0, 3

Before:

![2025-06-23-150055.png](/attachments/1eb92507-b8b4-422c-921c-4296a91172e7)

After:

![2025-06-23-150853.png](/attachments/c7b53397-54fe-487d-89da-e10b26538cde)

- [x] I did not document these changes and I do not expect someone else to do it.
- [x] I do not want this change to show in the release notes.

Co-authored-by: oliverpool <git@olivier.pfad.fr>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8439
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>
Co-committed-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>

models/forgejo_migrations/v36.go

@@ -0,0 +1,55 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: GPL-3.0-or-later
+
+package forgejo_migrations //nolint:revive
+
+import (
+	"xorm.io/xorm"
+)
+
+func FixWikiUnitDefaultPermission(x *xorm.Engine) error {
+	// Type is Unit's Type
+	type Type int
+
+	// Enumerate all the unit types
+	const (
+		TypeInvalid         Type = iota // 0 invalid
+		TypeCode                        // 1 code
+		TypeIssues                      // 2 issues
+		TypePullRequests                // 3 PRs
+		TypeReleases                    // 4 Releases
+		TypeWiki                        // 5 Wiki
+		TypeExternalWiki                // 6 ExternalWiki
+		TypeExternalTracker             // 7 ExternalTracker
+		TypeProjects                    // 8 Projects
+		TypePackages                    // 9 Packages
+		TypeActions                     // 10 Actions
+	)
+
+	// RepoUnitAccessMode specifies the users access mode to a repo unit
+	type UnitAccessMode int
+
+	const (
+		// UnitAccessModeUnset - no unit mode set
+		UnitAccessModeUnset UnitAccessMode = iota // 0
+		// UnitAccessModeNone no access
+		UnitAccessModeNone // 1
+		// UnitAccessModeRead read access
+		UnitAccessModeRead // 2
+		// UnitAccessModeWrite write access
+		UnitAccessModeWrite // 3
+	)
+	_ = UnitAccessModeNone
+	_ = UnitAccessModeWrite
+
+	type RepoUnit struct {
+		DefaultPermissions UnitAccessMode `xorm:"NOT NULL DEFAULT 0"`
+	}
+	_, err := x.Where("type = ?", TypeWiki).
+		Where("default_permissions = ?", UnitAccessModeRead).
+		Cols("default_permissions").
+		Update(RepoUnit{
+			DefaultPermissions: UnitAccessModeUnset,
+		})
+	return err
+}