feat(ugc): models with no bricks are "empty", not failed

A model with no bricks has nothing to make: it gets its own state
(is_optimized = 3), isn't counted as a failure or retried, shows as Empty
on the UGC page (with its own filter and count), in the status and in
Prometheus, and its downloads answer 404 like HKX. State names come from
the enum (magic_enum). Migrations dlu/mysql/87 and dlu/sqlite/70 move the
rows that failed only because they had no bricks.

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

dUgcServer/UgcJobs.h

@@ -40,6 +40,7 @@ namespace UgcJobs {
 		std::string note;        // what was odd but not fatal (missing bricks, ...)
 		UgcStorage::Files files; // name -> bytes, when ok
 		bool aoBaked{};
+		bool empty{};            // nothing to make (no bricks): not a failure
 		std::string stats;       // stats.json: bricks, triangles before and after per LOD, timings
 	};