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.EmulatedEntrypoint/Controllers/ArenaColosseumController.cs

@@ -688,33 +688,8 @@ public class ArenaColosseumController : SVSimController
 
     // --- helpers ---
 
-    private static ColosseumLobbyInfo BuildColosseumInfo(ColosseumSeasonConfig season)
-    {
-        if (!season.IsColosseumPeriod)
-        {
-            return new ColosseumLobbyInfo { IsColosseumPeriod = false };
-        }
-
-        return new ColosseumLobbyInfo
-        {
-            IsColosseumPeriod = true,
-            DeckFormat = (int)season.DeckFormat,
-            IsNormalTwoPick = season.IsNormalTwoPick ? "1" : "0",
-            ColosseumName = season.ColosseumName,
-            IsRoundPeriod = true,
-            IsSpecialMode = season.IsSpecialMode,
-            CardPoolName = string.IsNullOrEmpty(season.CardPoolName) ? null : season.CardPoolName,
-            NowRound = 1,
-            StartTime = FormatTime(season.EventStartTime),
-            EndTime = FormatTime(season.EventEndTime),
-            IsAllCardEnabled = season.IsAllCardEnabled ? 1 : 0,
-            SalesPeriodInfo = new SVSim.EmulatedEntrypoint.Models.Dtos.ColosseumSalesPeriodInfo
-            {
-                SalesPeriodTime = FormatTime(season.SalesPeriodEnd),
-            },
-            StrategyPickNum = season.StrategyPickNum > 0 ? season.StrategyPickNum : null,
-        };
-    }
+    private static ColosseumLobbyInfo BuildColosseumInfo(ColosseumSeasonConfig season) =>
+        ColosseumLobbyInfoBuilder.Build(season);
 
     /// <summary>Builds the <c>colosseum_status</c> block. When the viewer has no run, every
     /// property is null and global WhenWritingNull renders <c>{}</c> — the client