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/IBuildDeckRepository.cs

@@ -26,4 +26,11 @@ public interface IBuildDeckRepository
     /// Returns the new total.
     /// </summary>
     Task<int> IncrementPurchaseCount(long viewerId, int productId);
+
+    /// <summary>
+    /// Story deck-select decks for a class: StoryDeckEntry presentation rows joined to the matching
+    /// BuildDeckProductEntry card lists (deck_no == product_id), expanded to a flat card_id array.
+    /// Returns build and trial decks together; the caller splits by Kind.
+    /// </summary>
+    Task<List<StoryDeckView>> GetStoryDecksByClass(int classId);
 }