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/Controllers/BattlePassController.cs

@@ -28,4 +28,14 @@ public class BattlePassController : SVSimController
         if (info is null) return Ok(new { }); // off-season: empty payload
         return Ok(info);
     }
+
+    [HttpPost("item_list")]
+    public async Task<IActionResult> ItemList(BaseRequest request, CancellationToken ct)
+    {
+        if (!TryGetViewerId(out long viewerId)) return Unauthorized();
+
+        var list = await _battlePass.GetItemListAsync(viewerId, ct);
+        if (list is null) return Ok(new { });
+        return Ok(list);
+    }
 }