enhance(pull)!: add per-repository default squash merge commit message (#39816)

Adds a repository setting for the default squash merge commit message
body with GitHub's options `PR_BODY`, `COMMIT_MESSAGES` and `BLANK`,
also settable via `squash_merge_commit_message` in the API. The commit
title stays the pull request title.

1. Breaking: API squash merges without `merge_message_field` now get the
same default message as the web merge box instead of a title-only
commit, which affects clients sending only `Do` like Renovate. An empty
value keeps title-only.
2. Breaking: `[repository.pull-request] DEFAULT_SQUASH_MERGE_MESSAGE`
replaces `POPULATE_SQUASH_COMMENT_WITH_COMMIT_MESSAGES`, which still
works as a deprecated fallback. Repositories store the default when
created or next saved.
3. Deprecated app.ini keys now migrate through one table instead of
per-loader fallbacks, which also fixes the ignored `[mailer] CERT_FILE`
and `KEY_FILE` fallbacks. Deprecation warnings no longer claim a removal
version.

<img width="1891" height="571" alt="grafik"
src="https://github.com/user-attachments/assets/0fdcde13-1f11-4156-b598-d80eb0d04cbe"
/>

---------

Co-authored-by: silverwind <me@silverwind.io>

services/convert/repository.go

@@ -105,6 +105,7 @@ func innerToRepo(ctx context.Context, repo *repo_model.Repository, permissionInR
 	defaultDeleteBranchAfterMerge := false
 	defaultMergeStyle := repo_model.MergeStyleMerge
 	defaultUpdateStyle := repo_model.UpdateStyleMerge
+	defaultSquashMergeMessage := repo_model.DefaultPullRequestsConfig().DefaultSquashMergeMessage
 	defaultAllowMaintainerEdit := false
 	defaultTargetBranch := ""
 	if unit, err := repo.GetUnit(ctx, unit_model.TypePullRequests); err == nil {
@@ -123,6 +124,7 @@ func innerToRepo(ctx context.Context, repo *repo_model.Repository, permissionInR
 		defaultDeleteBranchAfterMerge = config.DefaultDeleteBranchAfterMerge
 		defaultMergeStyle = config.DefaultMergeStyle
 		defaultUpdateStyle = config.DefaultUpdateStyle
+		defaultSquashMergeMessage = config.DefaultSquashMergeMessage
 		defaultAllowMaintainerEdit = config.DefaultAllowMaintainerEdit
 		defaultTargetBranch = config.DefaultTargetBranch
 	}
@@ -251,6 +253,7 @@ func innerToRepo(ctx context.Context, repo *repo_model.Repository, permissionInR
 		DefaultDeleteBranchAfterMerge: defaultDeleteBranchAfterMerge,
 		DefaultMergeStyle:             string(defaultMergeStyle),
 		DefaultUpdateStyle:            string(defaultUpdateStyle),
+		SquashMergeCommitMessage:      string(defaultSquashMergeMessage),
 		DefaultAllowMaintainerEdit:    defaultAllowMaintainerEdit,
 		DefaultTargetBranch:           defaultTargetBranch,
 		AvatarURL:                     repo.AvatarLink(ctx),