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

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+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);
+}

SVSim.Database/Repositories/PackDrawTable/PackDrawTable.cs

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+using SVSim.Database.Models;
+
+namespace SVSim.Database.Repositories.PackDrawTables;
+
+/// <summary>
+/// All draw data for a single pack: per-pack config + slot rates + per-card weights.
+/// Loaded as one unit by <see cref="IPackDrawTableRepository.GetAsync"/>.
+/// </summary>
+public sealed class PackDrawTable
+{
+    public required PackDrawConfigEntry Config { get; init; }
+    public required IReadOnlyList<PackDrawSlotRateEntry> SlotRates { get; init; }
+    public required IReadOnlyList<PackDrawCardWeightEntry> CardWeights { get; init; }
+}

SVSim.Database/Repositories/PackDrawTable/PackDrawTableRepository.cs

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+using Microsoft.EntityFrameworkCore;
+
+namespace SVSim.Database.Repositories.PackDrawTables;
+
+public class PackDrawTableRepository : IPackDrawTableRepository
+{
+    private readonly SVSimDbContext _db;
+    public PackDrawTableRepository(SVSimDbContext db) { _db = db; }
+
+    public async Task<PackDrawTable?> GetAsync(int packId)
+    {
+        var config = await _db.PackDrawConfigs.FirstOrDefaultAsync(c => c.Id == packId);
+        if (config is null) return null;
+
+        var slotRates = await _db.PackDrawSlotRates
+            .Where(s => s.PackId == packId)
+            .ToListAsync();
+
+        var cardWeights = await _db.PackDrawCardWeights
+            .Where(w => w.PackId == packId)
+            .ToListAsync();
+
+        return new PackDrawTable
+        {
+            Config = config,
+            SlotRates = slotRates,
+            CardWeights = cardWeights,
+        };
+    }
+}