feat: Add default PR branch update style setting (#37410)

Adds repository-level settings for pull request branch updates so admins
can choose the default update method and disable merge or rebase
updates.

<img width="1025" height="158"
src="https://github.com/user-attachments/assets/d030973b-0ddd-4035-b04f-145c445084d7"
/>

---------

Co-authored-by: OpenAI Codex (GPT-5) <codex@openai.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/forms/repo_form.go

@@ -142,7 +142,9 @@ type RepoSettingForm struct {
 	PullsAllowManualMerge            bool
 	PullsDefaultMergeStyle           string
 	EnableAutodetectManualMerge      bool
+	PullsAllowMergeUpdate            bool
 	PullsAllowRebaseUpdate           bool
+	PullsDefaultUpdateStyle          string
 	DefaultDeleteBranchAfterMerge    bool
 	DefaultAllowMaintainerEdit       bool
 	DefaultTargetBranch              string