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>

migrations/dlu/sqlite/75_ugc_triangles_before.sql

@@ -0,0 +1,2 @@
+/* UGC server: a model's triangles before hidden faces were removed. See the MySQL migration. */
+ALTER TABLE ugc ADD COLUMN triangle_count_before INTEGER NOT NULL DEFAULT 0;