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>

routers/web/repo/setting/setting.go

@@ -680,6 +680,7 @@ func handleSettingsPostAdvanced(ctx *context.Context) {
 			DefaultUpdateStyle:            defaultUpdateStyle,
 			DefaultDeleteBranchAfterMerge: form.DefaultDeleteBranchAfterMerge,
 			DefaultMergeStyle:             repo_model.MergeStyle(form.PullsDefaultMergeStyle),
+			DefaultSquashMergeMessage:     util.IfZero(repo_model.SquashMergeMessage(form.PullsDefaultSquashMergeMessage), repo_model.DefaultPullRequestsConfig().DefaultSquashMergeMessage),
 			DefaultAllowMaintainerEdit:    form.DefaultAllowMaintainerEdit,
 			DefaultTargetBranch:           strings.TrimSpace(form.DefaultTargetBranch),
 		}
@@ -687,6 +688,10 @@ func handleSettingsPostAdvanced(ctx *context.Context) {
 			ctx.JSONErrorAuto(err)
 			return
 		}
+		if !prConfig.DefaultSquashMergeMessage.IsValid() {
+			ctx.JSONErrorAuto(util.NewInvalidArgumentErrorf("invalid default squash merge message: %s", prConfig.DefaultSquashMergeMessage))
+			return
+		}
 		units = append(units, newRepoUnit(repo, unit_model.TypePullRequests, prConfig))
 	} else if !unit_model.TypePullRequests.UnitGlobalDisabled() {
 		deleteUnitTypes = append(deleteUnitTypes, unit_model.TypePullRequests)