feat(ugc): how many triangles each model saved, on the UGC page

The UGC server stores a model's most detailed level's triangles before it
removed the faces that can't be seen (ugc.triangle_count_before;
migrations sqlite 75, mysql 92), from stats.json when it makes a model
and, once, for the models made before. The UGC page's models list has a
Saved column (the share and number of triangles removed, before/after in
its tooltip), sortable by the share (sort=savings), and the gallery can
sort by most triangles saved.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dUgcServer/UgcProcessor.h

@@ -187,6 +187,8 @@ private:
 	void Backfill();
 	std::deque<UgcStorage::Entry> m_BackfillItems;
 	bool m_BackfillItemsDone{ true };
+	std::deque<LWOOBJID> m_StatsBackfill; // models whose triangle counts are read from their stats.json once
+	bool m_StatsBackfillDone{ true };
 	bool m_BackfillBuildsDone{ false };
 
 	void Poll();