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]