feat(api): add last_sync to repository API (#37566)

This PR adds a new repository API field, `mirror_last_sync_at`, to
expose the timestamp of the last successful pull mirror sync.

Unlike `mirror_updated`, this field does not affect mirror scheduling
and is updated only after a successful pull sync. Failed sync attempts
leave the value unchanged.

What changed

- added `mirror_last_sync_at` to the repository API response
- updated pull mirror sync flow to persist the timestamp only on
successful sync
- kept `mirror_updated` behavior unchanged for queue/scheduling purposes

`mirror_updated` is currently tied to mirror queue behavior, so it
cannot safely represent the last successful sync time. The new field
makes that state explicit for API consumers without changing scheduling
semantics.

---------

Signed-off-by: pomidorry <106489913+Pomidorry@users.noreply.github.com>
Signed-off-by: wxiaoguang <wxiaoguang@gmail.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>
Co-authored-by: Giteabot <teabot@gitea.io>

services/mirror/mirror_pull.go

@@ -305,6 +305,7 @@ func SyncPullMirror(ctx context.Context, repoID int64) bool {
 
 	log.Trace("SyncMirrors [repo: %-v]: Scheduling next update", m.Repo)
 	m.ScheduleNextUpdate()
+	m.LastSyncUnix = m.UpdatedUnix
 	if err = repo_model.UpdateMirror(ctx, m); err != nil {
 		log.Error("SyncMirrors [repo: %-v]: failed to UpdateMirror with next update date: %v", m.Repo, err)
 		return false