feat(bp): /battle_pass/item_list — derives product per active season

Adds BattlePassSalesPeriodInfoDto, BattlePassProductDto, BattlePassItemListResponse DTOs,
GetItemListAsync on BattlePassService (one product if not premium + CanPurchase, empty if
already premium or off-season), and the /battle_pass/item_list controller action.
2 new integration tests; all 408 pass.

SVSim.EmulatedEntrypoint/Services/IBattlePassService.cs

@@ -13,4 +13,10 @@ public interface IBattlePassService
     /// (controller emits empty body in that case).
     /// </summary>
     Task<BattlePassInfoResponse?> GetInfoAsync(long viewerId, CancellationToken ct);
+
+    /// <summary>
+    /// /battle_pass/item_list payload. Returns one product per active season; empty products
+    /// array if the viewer already owns premium for the active season. Null when no active season.
+    /// </summary>
+    Task<BattlePassItemListResponse?> GetItemListAsync(long viewerId, CancellationToken ct);
 }