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/SVSim.Database.csproj

@@ -6,6 +6,10 @@
         <Nullable>enable</Nullable>
     </PropertyGroup>
 
+    <ItemGroup>
+      <InternalsVisibleTo Include="SVSim.UnitTests" />
+    </ItemGroup>
+
     <ItemGroup>
       <PackageReference Include="CsvHelper" Version="33.0.1" />
       <PackageReference Include="Microsoft.EntityFrameworkCore" Version="8.0.8" />