refactor(arena-colosseum): dedup colosseum_info source-of-truth

/mypage/index data.colosseum_info and /arena_colosseum/{top,get_fee_info}
now both project through ColosseumLobbyInfoBuilder.Build(ColosseumSeasonConfig).
One source of truth for the home-tab gating and the lobby reads — admins
flip the season on by writing to GameConfigs (per the runbook), no second
table to keep in sync.

* New ColosseumLobbyInfoBuilder — static projection from ColosseumSeasonConfig
  to the wire ColosseumLobbyInfo. Replaces ArenaColosseumController's private
  BuildColosseumInfo and MyPageController's BuildColosseumInfo(ColosseumConfig).
* MyPageController reads ColosseumSeasonConfig via IGameConfigService; the
  IGlobalsRepository.GetCurrentColosseum() call goes away.
* MyPageIndexResponse.ColosseumInfo retyped to the new ColosseumLobbyInfo.
* ColosseumSalesPeriodInfo relocated into the ArenaColosseum namespace
  alongside the rest of the family DTOs.
* Drops: Colosseums DB table (migration DropColosseumsTable), ColosseumConfig
  entity, the captured-shape Models.Dtos.ColosseumInfo DTO, ColosseumSeed,
  MyPageGlobalsImporter.ImportColosseumAsync + seed file colosseum.json,
  IGlobalsRepository.GetCurrentColosseum + impl, the two related tests.

Net: home-screen Grand Prix tab now hidden by default because the season
ships with IsColosseumPeriod=false. Full suite: 1345/1345 (was 1347 — the
2 dropped tests covered the now-dead surface).

SVSim.Bootstrap/Program.cs

@@ -114,7 +114,6 @@ public static class Program
 
             var mypage = new MyPageGlobalsImporter();
             await mypage.ImportBannersAsync(context, opts.SeedDir);
-            await mypage.ImportColosseumAsync(context, opts.SeedDir);
             await mypage.ImportSealedAsync(context, opts.SeedDir);
             await mypage.ImportMasterPointRankingPeriodAsync(context, opts.SeedDir);
             await mypage.ImportSpecialDeckFormatsAsync(context, opts.SeedDir);