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/UgcProcessor.h

@@ -189,7 +189,8 @@ private:
 	std::set<LWOOBJID> m_ComboJobs;                         // combinations being made
 	std::map<LWOOBJID, std::vector<std::pair<LWOOBJID, uint32_t>>> m_ComboRows; // combination -> builds (id, attempts) waiting for it
 	std::map<LWOOBJID, LWOOBJID> m_ComboOf;                 // build -> combination (StorageId)
-	uint64_t m_Reused{};                                    // builds whose combination was made already
+	uint64_t m_Reused{};
+	uint64_t m_Empty{};  // models with no bricks (nothing to make)                                    // builds whose combination was made already
 	std::chrono::steady_clock::time_point m_NextPoll{};
 	std::chrono::steady_clock::time_point m_NextEviction{};
 	std::map<std::pair<Kind, LWOOBJID>, std::pair<std::chrono::steady_clock::time_point, Availability>> m_Recent; // answers for missing files