feat(repo): GetStoryDecksByClass joins story-deck presentation to product card lists

Adds StoryDeckView projection, IBuildDeckRepository.GetStoryDecksByClass interface method,
and BuildDeckRepository implementation that loads StoryDeckEntry rows for a class, fetches
matching BuildDeckProductEntry card lists, and expands each card by Number into a flat
CardIdArray. TDD: 2 tests in StoryDeckRepositoryTests (expand + empty-class).

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

SVSim.Database/Repositories/BuildDeck/BuildDeckRepository.cs

@@ -64,4 +64,38 @@ public class BuildDeckRepository : IBuildDeckRepository
         await _db.SaveChangesAsync();
         return row.PurchaseCount;
     }
+
+    public async Task<List<StoryDeckView>> GetStoryDecksByClass(int classId)
+    {
+        var decks = await _db.StoryDecks.Where(d => d.ClassId == classId).ToListAsync();
+        if (decks.Count == 0) return new();
+
+        var ids = decks.Select(d => d.DeckNo).ToList();
+        var products = await _db.BuildDeckProducts
+            .Where(p => ids.Contains(p.Id))
+            .Include(p => p.Cards)
+            .AsSplitQuery()
+            .ToListAsync();
+
+        // Expand each product's owned card rows by Number into a flat card_id list (spots included —
+        // validated against the prod capture, 112/112 match).
+        var cardsById = products.ToDictionary(
+            p => p.Id,
+            p => p.Cards.SelectMany(c => Enumerable.Repeat(c.CardId, c.Number)).ToList());
+
+        return decks.Select(d => new StoryDeckView
+        {
+            DeckNo = d.DeckNo,
+            Kind = d.Kind,
+            ClassId = d.ClassId,
+            DeckName = d.DeckName,
+            SleeveId = d.SleeveId,
+            LeaderSkinId = d.LeaderSkinId,
+            IsRecommend = d.IsRecommend,
+            OrderNum = d.OrderNum,
+            EntryNo = d.EntryNo,
+            DeckFormat = d.DeckFormat,
+            CardIdArray = cardsById.TryGetValue(d.DeckNo, out var cards) ? cards : new(),
+        }).ToList();
+    }
 }