Add DEFAULT_DELETE_BRANCH_AFTER_MERGE setting (#36917)

Add this config option, applying to new repos:

```ini
[repository.pull-request]
DEFAULT_DELETE_BRANCH_AFTER_MERGE = true
```

Defaults to `false`, preserving current behavior.

---------

Co-authored-by: Claude (Opus 4.6) <noreply@anthropic.com>

custom/conf/app.example.ini

@@ -1162,6 +1162,9 @@ LEVEL = Info
 ;; Delay mergeable check until page view or API access, for pull requests that have not been updated in the specified days when their base branches get updated.
 ;; Use "-1" to always check all pull requests (old behavior). Use "0" to always delay the checks.
 ;DELAY_CHECK_FOR_INACTIVE_DAYS = 7
+;;
+;; Set the default value for "Delete pull request branch after merge by default" for new repositories
+;DEFAULT_DELETE_BRANCH_AFTER_MERGE = false
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

models/repo/repo_unit.go

@@ -142,6 +142,7 @@ func DefaultPullRequestsConfig() *PullRequestsConfig {
 		AllowRebaseUpdate:          true,
 		DefaultAllowMaintainerEdit: true,
 	}
+	cfg.DefaultDeleteBranchAfterMerge = setting.Repository.PullRequest.DefaultDeleteBranchAfterMerge
 	cfg.DefaultMergeStyle = MergeStyle(setting.Repository.PullRequest.DefaultMergeStyle)
 	cfg.DefaultMergeStyle = util.IfZero(cfg.DefaultMergeStyle, MergeStyleMerge)
 	return cfg

modules/setting/repository.go

@@ -89,6 +89,7 @@ var (
 			TestConflictingPatchesWithGitApply       bool
 			RetargetChildrenOnMerge                  bool
 			DelayCheckForInactiveDays                int
+			DefaultDeleteBranchAfterMerge            bool
 		} `ini:"repository.pull-request"`
 
 		// Issue Setting
@@ -213,6 +214,7 @@ var (
 			TestConflictingPatchesWithGitApply       bool
 			RetargetChildrenOnMerge                  bool
 			DelayCheckForInactiveDays                int
+			DefaultDeleteBranchAfterMerge            bool
 		}{
 			WorkInProgressPrefixes: []string{"WIP:", "[WIP]"},
 			// Same as GitHub. See