feat(bp): /battle_pass/buy — crystal-cost + retroactive premium grants

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Controllers/BattlePassController.cs

@@ -38,4 +38,35 @@ public class BattlePassController : SVSimController
         if (list is null) return Ok(new { });
         return Ok(list);
     }
+
+    [HttpPost("buy")]
+    public async Task<IActionResult> Buy(BattlePassBuyRequest request, CancellationToken ct)
+    {
+        if (!TryGetViewerId(out long viewerId)) return Unauthorized();
+
+        var outcome = await _battlePass.BuyPremiumAsync(viewerId, request.SeasonId, request.Id, ct);
+
+        var response = new BattlePassBuyResponse
+        {
+            ResultCode = outcome.ResultCode,
+            AchievedInfo = new BattlePassAchievedInfoDto
+            {
+                BattlePassRewardList = outcome.AchievedRewards
+                    .Select(g => new BattlePassReceivedRewardDto
+                    {
+                        RewardType = g.RewardType,
+                        RewardDetailId = g.RewardId,
+                        RewardNumber = g.RewardNum,
+                    }).ToList(),
+            },
+            RewardList = outcome.PostStateTotals
+                .Select(g => new BattlePassRewardListEntryDto
+                {
+                    RewardType = g.RewardType,
+                    RewardId = g.RewardId,
+                    RewardNum = g.RewardNum,
+                }).ToList(),
+        };
+        return Ok(response);
+    }
 }