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>

modules/structs/repo.go

@@ -125,10 +125,12 @@ type Repository struct {
 	// ObjectFormatName of the underlying git repository
 	ObjectFormatName ObjectFormatName `json:"object_format_name"`
 	// swagger:strfmt date-time
-	MirrorUpdated time.Time     `json:"mirror_updated"`
-	RepoTransfer  *RepoTransfer `json:"repo_transfer,omitempty"`
-	Topics        []string      `json:"topics"`
-	Licenses      []string      `json:"licenses"`
+	MirrorUpdated time.Time `json:"mirror_updated"`
+	// swagger:strfmt date-time
+	MirrorLastSyncAt time.Time     `json:"mirror_last_sync_at"`
+	RepoTransfer     *RepoTransfer `json:"repo_transfer,omitempty"`
+	Topics           []string      `json:"topics"`
+	Licenses         []string      `json:"licenses"`
 }
 
 // CreateRepoOption options when creating repository