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>

models/migrations/migrations.go

@@ -409,6 +409,7 @@ func prepareMigrationTasks() []*migration {
 		// Gitea 1.26.0 ends at migration ID number 330 (database version 331)
 
 		newMigration(331, "Add ActionRunAttempt model and related action fields", v1_27.AddActionRunAttemptModel),
+		newMigration(332, "Add last_sync_unix to mirror", v1_27.AddLastSyncUnixToMirror),
 	}
 	return preparedMigrations
 }