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.EmulatedEntrypoint/Models/Dtos/Responses/IndexResponse.cs

@@ -243,7 +243,7 @@ public class IndexResponse
     /// </summary>
     [JsonPropertyName("battle_pass_level_info")]
     [Key("battle_pass_level_info")]
-    public Dictionary<string, BattlePassLevel>? BattlePassLevelInfo { get; set; }
+    public IReadOnlyDictionary<string, BattlePassLevel>? BattlePassLevelInfo { get; set; }
 
     /// <summary>
     /// Wire is string[]; parser calls .ToString() on each element (LoadDetail.cs:493-499).