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>

templates/swagger/v1_json.tmpl

@@ -29039,6 +29039,11 @@
           "type": "string",
           "x-go-name": "MirrorInterval"
         },
+        "mirror_last_sync_at": {
+          "type": "string",
+          "format": "date-time",
+          "x-go-name": "MirrorLastSyncAt"
+        },
         "mirror_updated": {
           "type": "string",
           "format": "date-time",

templates/swagger/v1_openapi3_json.tmpl

@@ -9292,6 +9292,11 @@
             "type": "string",
             "x-go-name": "MirrorInterval"
           },
+          "mirror_last_sync_at": {
+            "format": "date-time",
+            "type": "string",
+            "x-go-name": "MirrorLastSyncAt"
+          },
           "mirror_updated": {
             "format": "date-time",
             "type": "string",