refactor: clean up git repo and model migration packages (#38564)

enable the golangci depguard lint rule: deny "models" and its sub
packages in "modelmigration" package.

modelmigration/base/db.go

@@ -11,19 +11,14 @@ import (
 	"regexp"
 	"strings"
 
-	"gitea.dev/models/db"
+	"gitea.dev/models/db" //nolint:depguard // allow to access db in migration
 	"gitea.dev/modules/log"
 	"gitea.dev/modules/setting"
 
+	"xorm.io/builder"
 	"xorm.io/xorm/schemas"
 )
 
-// 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".
-// However, many the existing migrations already abuses "modules/structs" (search "gitea.dev/models/" in the migrations).
-// TODO: need to fully decouple the migration package and models & structs package
 type (
 	EngineMigration = db.EngineMigration
 	Session         = db.Session
@@ -519,3 +514,7 @@ func ModifyColumn(x EngineMigration, tableName string, col *schemas.Column) erro
 	}
 	return nil
 }
+
+func Iterate[Bean any](ctx context.Context, cond builder.Cond, f func(ctx context.Context, bean *Bean) error) error {
+	return db.Iterate(ctx, cond, f)
+}

modelmigration/base/git.go

@@ -1,13 +0,0 @@
-// Copyright 2026 The Gitea Authors. All rights reserved.
-// SPDX-License-Identifier: MIT
-
-package base
-
-import (
-	repo_model "gitea.dev/models/repo"
-	"gitea.dev/modules/git/gitcmd"
-)
-
-func LocalCodeGitRepo(ownerName, repoName string) gitcmd.RepositoryFacade {
-	return repo_model.CodeRepoByName(ownerName, repoName)
-}

modelmigration/base/shared.go

@@ -0,0 +1,25 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package base
+
+import (
+	"gitea.dev/models/db" //nolint:depguard // allow to access db in migration
+	"gitea.dev/modules/git/gitrepo"
+)
+
+// HINT: MIGRATION-STRUCT-FROZEN: the structs used in migrations should not be affected by the changes happen in the future,
+// because the details 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 existing migration will break because it will use the new (incorrect) User model,
+// it should only use the old User model.
+//
+// Related: "models", "modules/structs", git repo directory layout on the disk, etc.
+//
+// If changes happen, the old migrations need to use a snapshot struct/function (copy the code and freeze)
+
+type ResourceIndex = db.ResourceIndex
+
+func LocalCodeGitRepo(ownerName, repoName string) gitrepo.RepositoryFacade {
+	return gitrepo.CodeRepoByName(ownerName, repoName)
+}