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/Models/Dtos/Responses/MyPageIndexResponse.cs

@@ -1,5 +1,6 @@
 using MessagePack;
 using SVSim.Database.Enums;
+using SVSim.EmulatedEntrypoint.Models.Dtos.ArenaColosseum;
 using System.Text.Json.Serialization;
 
 namespace SVSim.EmulatedEntrypoint.Models.Dtos.Responses;
@@ -134,11 +135,13 @@ public class MyPageIndexResponse
     /// <summary>
     /// Required — ColosseumEntryInfoTask.SetColosseumInfo indexes this key
     /// directly (Wizard/ColosseumEntryInfoTask.cs:102) and reads
-    /// is_colosseum_period without a guard.
+    /// is_colosseum_period without a guard. Built by
+    /// <see cref="ColosseumLobbyInfoBuilder"/> from <c>ColosseumSeasonConfig</c> —
+    /// shared with <c>/arena_colosseum/{top,get_fee_info}</c>.
     /// </summary>
     [JsonPropertyName("colosseum_info")]
     [Key("colosseum_info")]
-    public ColosseumInfo ColosseumInfo { get; set; } = new();
+    public ColosseumLobbyInfo ColosseumInfo { get; set; } = new();
 
     // ── Convention / offline event ─────────────────────────────────────────