feat: keep every brick by brick save before it is split into models

The save is stored as the client sent it in the new ugc_unsplit table, and each model split from it points back to it (ugc.unsplit_id), so the whole build can be looked at or restored later. A save is kept while any model split from it exists: deleting models removes saves no model points to any more, and deleting a character removes its saves.

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

migrations/dlu/mysql/27_ugc_unsplit.sql

@@ -0,0 +1,11 @@
+-- Every brick by brick save as the client sent it (sd0), before the server splits it into one model per group of
+-- connected bricks; each model made from it points back to it
+CREATE TABLE IF NOT EXISTS ugc_unsplit (
+	id BIGINT NOT NULL PRIMARY KEY,
+	account_id INT NOT NULL REFERENCES accounts(id),
+	character_id BIGINT NOT NULL REFERENCES charinfo(id),
+	lxfml MEDIUMBLOB NOT NULL,
+	created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+ALTER TABLE ugc ADD COLUMN unsplit_id BIGINT NULL DEFAULT NULL;