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/PackDrawSlotRateSeed.cs

@@ -0,0 +1,11 @@
+using System.Text.Json.Serialization;
+
+namespace SVSim.Bootstrap.Models.Seed;
+
+public sealed class PackDrawSlotRateSeed
+{
+    [JsonPropertyName("pack_id")] public int PackId { get; set; }
+    [JsonPropertyName("slot")] public string Slot { get; set; } = "general";
+    [JsonPropertyName("tier")] public string Tier { get; set; } = "bronze";
+    [JsonPropertyName("rate_pct")] public double RatePct { get; set; }
+}