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/ArenaColosseum/ColosseumInfo.cs

@@ -1,6 +1,5 @@
 using System.Text.Json.Serialization;
 using MessagePack;
-using SVSim.EmulatedEntrypoint.Models.Dtos;
 
 namespace SVSim.EmulatedEntrypoint.Models.Dtos.ArenaColosseum;
 
@@ -10,10 +9,11 @@ namespace SVSim.EmulatedEntrypoint.Models.Dtos.ArenaColosseum;
 /// <see cref="IsColosseumPeriod"/> is <c>false</c>, the client skips parsing every other
 /// field — server still emits this minimal payload so the lobby renders cleanly.
 /// <para>
-/// Distinct from <see cref="SVSim.EmulatedEntrypoint.Models.Dtos.ColosseumInfo"/> (the
-/// captured prod <c>/mypage/index</c> shape — stringly-typed and read-only). This is the
-/// per-endpoint Colosseum-family shape. <c>/event_info</c> uses yet a third shape —
-/// <see cref="ColosseumEventInfo"/>.
+/// Also drives <c>/mypage/index data.colosseum_info</c> — the home-screen tab gating and
+/// the per-endpoint lobby reads share one source of truth (the
+/// <see cref="ColosseumSeasonConfig"/> POCO) projected via
+/// <see cref="ColosseumLobbyInfoBuilder"/>. <c>/event_info</c> uses a different,
+/// event-level shape — <see cref="ColosseumEventInfo"/>.
 /// </para>
 /// </summary>
 [MessagePackObject]

SVSim.EmulatedEntrypoint/Models/Dtos/ArenaColosseum/ColosseumLobbyInfoBuilder.cs

@@ -0,0 +1,48 @@
+using System.Globalization;
+using SVSim.Database.Models.Config;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.ArenaColosseum;
+
+/// <summary>
+/// Single source of truth for projecting <see cref="ColosseumSeasonConfig"/> onto the wire
+/// <see cref="ColosseumLobbyInfo"/> block. Used by both <c>/arena_colosseum/{top,get_fee_info}</c>
+/// AND <c>/mypage/index data.colosseum_info</c> — the home-screen tab and the lobby reads
+/// must agree on the season state, so they go through one builder. When
+/// <c>IsColosseumPeriod = false</c> the minimal "no event" payload is emitted; the client
+/// gates every other field on that flag (<c>Wizard/ColosseumEntryInfoTask.cs:100</c>).
+/// </summary>
+public static class ColosseumLobbyInfoBuilder
+{
+    private const string WireDateFormat = "yyyy-MM-dd HH:mm:ss";
+
+    public static ColosseumLobbyInfo Build(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 ColosseumSalesPeriodInfo
+            {
+                SalesPeriodTime = FormatTime(season.SalesPeriodEnd),
+            },
+            StrategyPickNum = season.StrategyPickNum > 0 ? season.StrategyPickNum : null,
+        };
+    }
+
+    private static string FormatTime(DateTime t) =>
+        t == default ? "" : t.ToString(WireDateFormat, CultureInfo.InvariantCulture);
+}

SVSim.EmulatedEntrypoint/Models/Dtos/ArenaColosseum/ColosseumSalesPeriodInfo.cs

@@ -0,0 +1,16 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.ArenaColosseum;
+
+/// <summary>
+/// Nested under <c>colosseum_info.sales_period_info</c>. Captured prod shape — single
+/// <c>sales_period_time</c> field carrying the wall-clock end of the cup's sales window.
+/// Format <c>"yyyy-MM-dd HH:mm:ss"</c> (PHP convention, not ISO).
+/// </summary>
+[MessagePackObject]
+public sealed class ColosseumSalesPeriodInfo
+{
+    [JsonPropertyName("sales_period_time")] [Key("sales_period_time")]
+    public string SalesPeriodTime { get; set; } = string.Empty;
+}