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.Database/SVSimDbContext.cs

@@ -63,7 +63,6 @@ public class SVSimDbContext : DbContext
     public DbSet<DailyLoginBonusEntry> DailyLoginBonuses => Set<DailyLoginBonusEntry>();
     public DbSet<BannerEntry> Banners => Set<BannerEntry>();
     public DbSet<HomeDialogEntry> HomeDialogEntries => Set<HomeDialogEntry>();
-    public DbSet<ColosseumConfig> Colosseums => Set<ColosseumConfig>();
     public DbSet<SealedConfig> SealedSeasons => Set<SealedConfig>();
     public DbSet<MasterPointRankingPeriodEntry> MasterPointRankingPeriods => Set<MasterPointRankingPeriodEntry>();
     public DbSet<SpecialDeckFormatEntry> SpecialDeckFormats => Set<SpecialDeckFormatEntry>();