review(bp): drop fragile cast, thread CT, internalize cache reset

- IndexResponse.BattlePassLevelInfo widened to IReadOnlyDictionary<string,BattlePassLevel>?
  so any IReadOnlyDictionary impl (FrozenDictionary, wrapper, etc.) serializes correctly
  instead of silently null-ing via a failed as-cast
- LoadController.Index now takes CancellationToken ct and threads it to GetLevelCurveAsync
  instead of CancellationToken.None
- BattlePassRepository.ResetLevelCurveCache changed from public to internal; added
  InternalsVisibleTo("SVSim.UnitTests") to SVSim.Database.csproj (was absent)

SVSim.Database/Repositories/BattlePass/BattlePassRepository.cs

@@ -56,5 +56,5 @@ public sealed class BattlePassRepository : IBattlePassRepository
     /// cache has already been populated (by an earlier test's HTTP call) must call this before
     /// re-seeding so the next read fetches fresh rows.
     /// </summary>
-    public static void ResetLevelCurveCache() => _curveCache = null;
+    internal static void ResetLevelCurveCache() => _curveCache = null;
 }