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.Bootstrap/Data/seeds/colosseum.json

@@ -1,20 +0,0 @@
-{
-  "id": 1,
-  "colosseum_id": "165",
-  "colosseum_name": "Rivenbrandt Take Two Cup",
-  "card_pool_name": "Take Two (Dragonblade–Rivenbrandt)",
-  "deck_format": "3",
-  "start_time": "2026-05-21 06:00:00",
-  "end_time": "2026-05-25 19:59:59",
-  "now_round": "1",
-  "is_display_tips": "0",
-  "tips_id": "0",
-  "is_colosseum_period": true,
-  "is_round_period": true,
-  "is_normal_two_pick": "1",
-  "is_special_mode": "10",
-  "is_all_card_enabled": 0,
-  "sales_period_info": {
-    "sales_period_time": "2026-05-25 19:59:59"
-  }
-}