feat(packs): PackDrawTable aggregate + IPackDrawTableRepository

Aggregate (Config + SlotRates + CardWeights) and a single-pack getter
loaded as one unit per /pack/open. PackOpenService consumes the
aggregate; tests use the production seed (fixture overlay) to validate
shape.

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

SVSim.Database/Repositories/PackDrawTable/IPackDrawTableRepository.cs

@@ -0,0 +1,7 @@
+namespace SVSim.Database.Repositories.PackDrawTables;
+
+public interface IPackDrawTableRepository
+{
+    /// <summary>Returns the draw table for <paramref name="packId"/>, or null if not seeded.</summary>
+    Task<PackDrawTable?> GetAsync(int packId);
+}