feat(pack): /pack/exchange_gacha_point endpoint

Wires IGachaPointService.TryExchangeAsync into a controller endpoint.
Loads the viewer with the full cosmetic-grant graph (Cards, Sleeves,
Emblems, Degrees, LeaderSkins, MyPageBackgrounds) plus the gacha-point
balance + received marker, with AsSplitQuery to avoid the cartesian
explosion documented in project_ef_split_query. Returns BadRequest with
the outcome's error code on validation failure, or the post-state
reward_list on success.

Two integration tests: happy-path verifies card grant + balance debit +
received-marker persistence + post-state reward_list shape; insufficient-
balance path returns 400.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Controllers/PackController.cs

@@ -163,6 +163,37 @@ public class PackController : SVSimController
         };
     }
 
+    [HttpPost("exchange_gacha_point")]
+    public async Task<ActionResult<ExchangeGachaPointResponse>> ExchangeGachaPoint(
+        ExchangeGachaPointRequest request)
+    {
+        if (!TryGetViewerId(out long viewerId)) return Unauthorized();
+
+        // Load the viewer with the collections the service mutates (balances, received marker,
+        // cards, cosmetics). AsSplitQuery per project_ef_split_query memory.
+        var viewer = await _db.Viewers
+            .Include(v => v.GachaPointBalances)
+            .Include(v => v.GachaPointReceived)
+            .Include(v => v.Cards)
+            .Include(v => v.Sleeves)
+            .Include(v => v.Emblems)
+            .Include(v => v.Degrees)
+            .Include(v => v.LeaderSkins)
+            .Include(v => v.MyPageBackgrounds)
+            .AsSplitQuery()
+            .FirstAsync(v => v.Id == viewerId);
+
+        var outcome = await _gachaPoint.TryExchangeAsync(viewer, request.ParentGachaId, request.CardId);
+        if (!outcome.Success) return BadRequest(new { error = outcome.Error });
+
+        await _db.SaveChangesAsync();
+
+        return new ExchangeGachaPointResponse
+        {
+            RewardList = outcome.RewardList.ToList(),
+        };
+    }
+
     [HttpPost("open")]
     [HttpPost("/tutorial/pack_open")]
     public async Task<ActionResult<PackOpenResponse>> Open(PackOpenRequest request)

SVSim.UnitTests/Controllers/PackControllerGachaPointTests.cs

@@ -112,4 +112,112 @@ public class PackControllerGachaPointTests
         Assert.That(text, Does.Contain("\"is_received\":false"));
         Assert.That(text, Does.Contain("\"is_display_prize\":false"));
     }
+
+    [Test]
+    public async Task ExchangeGachaPoint_grants_card_and_returns_post_state_reward_list()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        using (var scope = factory.Services.CreateScope())
+        {
+            var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            db.Classes.Add(new ClassEntry { Id = 0, Name = "Neutral" });
+            var set = new ShadowverseCardSetEntry { Id = 10008, IsInRotation = true };
+            db.CardSets.Add(set);
+            set.Cards.Add(new ShadowverseCardEntry
+            {
+                Id = 108041010, Name = "leg", Rarity = Rarity.Legendary,
+                Class = db.Classes.Local.First(), IsFoil = false,
+            });
+            db.CardCosmeticRewards.Add(new CardCosmeticReward
+            {
+                CardId = 108041010, Type = CosmeticType.Emblem, CosmeticId = 1080410100,
+            });
+            db.Packs.Add(new PackConfigEntry
+            {
+                Id = 10008, BasePackId = 10008, PackCategory = PackCategory.LegendCardPack,
+                CommenceDate = DateTime.UtcNow.AddDays(-1), CompleteDate = DateTime.UtcNow.AddDays(30),
+                GachaPointConfig = new PackGachaPointConfig { ExchangeablePoint = 400, IncreaseGachaPoint = 1 },
+            });
+            var viewer = await db.Viewers
+                .Include(v => v.GachaPointBalances)
+                .FirstAsync(v => v.Id == viewerId);
+            viewer.GachaPointBalances.Add(new ViewerGachaPointBalance { PackId = 10008, Points = 500 });
+            await db.SaveChangesAsync();
+        }
+
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+        var body = JsonBody("""{"card_id":108041010,"parent_gacha_id":10008,"odds_gacha_id":10008,"viewer_id":"0","steam_id":0,"steam_session_ticket":""}""");
+        var response = await client.PostAsync("/pack/exchange_gacha_point", body);
+        var text = await response.Content.ReadAsStringAsync();
+
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK), text);
+
+        using var doc = JsonDocument.Parse(text);
+        var rewardList = doc.RootElement.GetProperty("reward_list");
+        Assert.That(rewardList.GetArrayLength(), Is.GreaterThan(0));
+        // Verify the card grant entry (type=5/Card) is present with the granted card id.
+        bool foundCard = false;
+        foreach (var r in rewardList.EnumerateArray())
+        {
+            if (r.GetProperty("reward_type").GetInt32() == 5 &&
+                r.GetProperty("reward_id").GetInt64() == 108041010)
+            {
+                foundCard = true;
+                break;
+            }
+        }
+        Assert.That(foundCard, Is.True, "card grant entry missing from reward_list");
+
+        // Verify side-effects.
+        using (var scope = factory.Services.CreateScope())
+        {
+            var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            var viewer = await db.Viewers
+                .Include(v => v.GachaPointBalances)
+                .Include(v => v.GachaPointReceived)
+                .Include(v => v.Cards).ThenInclude(c => c.Card)
+                .AsSplitQuery()
+                .FirstAsync(v => v.Id == viewerId);
+            Assert.That(viewer.GachaPointBalances.Single().Points, Is.EqualTo(100));
+            Assert.That(viewer.GachaPointReceived.Single().CardId, Is.EqualTo(108041010));
+            Assert.That(viewer.Cards.Any(c => c.Card.Id == 108041010), Is.True);
+        }
+    }
+
+    [Test]
+    public async Task ExchangeGachaPoint_rejects_when_balance_insufficient()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        using (var scope = factory.Services.CreateScope())
+        {
+            var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+            db.Classes.Add(new ClassEntry { Id = 0, Name = "Neutral" });
+            var set = new ShadowverseCardSetEntry { Id = 10008, IsInRotation = true };
+            db.CardSets.Add(set);
+            set.Cards.Add(new ShadowverseCardEntry
+            {
+                Id = 108041010, Name = "leg", Rarity = Rarity.Legendary,
+                Class = db.Classes.Local.First(), IsFoil = false,
+            });
+            db.CardCosmeticRewards.Add(new CardCosmeticReward
+            {
+                CardId = 108041010, Type = CosmeticType.Emblem, CosmeticId = 1080410100,
+            });
+            db.Packs.Add(new PackConfigEntry
+            {
+                Id = 10008, BasePackId = 10008, PackCategory = PackCategory.LegendCardPack,
+                CommenceDate = DateTime.UtcNow.AddDays(-1), CompleteDate = DateTime.UtcNow.AddDays(30),
+                GachaPointConfig = new PackGachaPointConfig { ExchangeablePoint = 400, IncreaseGachaPoint = 1 },
+            });
+            await db.SaveChangesAsync();
+        }
+
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+        var body = JsonBody("""{"card_id":108041010,"parent_gacha_id":10008,"odds_gacha_id":10008,"viewer_id":"0","steam_id":0,"steam_session_ticket":""}""");
+        var response = await client.PostAsync("/pack/exchange_gacha_point", body);
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.BadRequest),
+            await response.Content.ReadAsStringAsync());
+    }
 }