feat(packs): add PackDraw seed DTOs and IsEnabled on PackSeed

Three new seed DTOs (PackDrawConfigSeed, PackDrawSlotRateSeed,
PackDrawCardWeightSeed) — slot and tier carried as strings, importer
translates to enum.

PackSeed gains is_enabled (defaults true so existing seeds remain
enabled).

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

SVSim.Bootstrap/Models/Seed/PackSeed.cs

@@ -24,6 +24,7 @@ public sealed class PackSeed
     [JsonPropertyName("gacha_point")] public PackGachaPointSeed? GachaPoint { get; set; }
     [JsonPropertyName("child_gachas")] public List<PackChildGachaSeed> ChildGachas { get; set; } = new();
     [JsonPropertyName("banners")] public List<PackBannerSeed> Banners { get; set; } = new();
+    [JsonPropertyName("is_enabled")] public bool IsEnabled { get; set; } = true;
 }
 
 public sealed class PackGachaPointSeed