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>

tests/integration/mirror_pull_test.go

@@ -93,6 +93,9 @@ func TestMirrorPull(t *testing.T) {
 	ok := mirror_service.SyncPullMirror(ctx, mirrorRepo.ID)
 	assert.True(t, ok)
 
+	mirror := unittest.AssertExistsAndLoadBean(t, &repo_model.Mirror{RepoID: mirrorRepo.ID})
+	assert.Equal(t, mirror.UpdatedUnix, mirror.LastSyncUnix)
+
 	// actually there is a tag in the source repo, so after "sync", that tag will also come into the mirror
 	initCount++
 
@@ -110,4 +113,14 @@ func TestMirrorPull(t *testing.T) {
 	count, err = db.Count[repo_model.Release](t.Context(), findOptions)
 	assert.NoError(t, err)
 	assert.Equal(t, initCount, count)
+
+	mirror = unittest.AssertExistsAndLoadBean(t, &repo_model.Mirror{RepoID: mirrorRepo.ID})
+	lastMirrorSync := mirror.LastSyncUnix
+	assert.NoError(t, mirror_service.UpdateAddress(ctx, mirror, repoPath+"-missing"))
+
+	ok = mirror_service.SyncPullMirror(ctx, mirrorRepo.ID)
+	assert.False(t, ok)
+
+	mirror = unittest.AssertExistsAndLoadBean(t, &repo_model.Mirror{RepoID: mirrorRepo.ID})
+	assert.Equal(t, lastMirrorSync, mirror.LastSyncUnix)
 }