feat(tk2): weighted-group reward picking

Replaces the all-rows-granted reward model with per-group weighted
pick. Each ArenaTwoPickReward row now belongs to a RewardGroup with a
Weight; finish/retire groups the WinCount's rows by RewardGroup and
picks exactly one row per group, weighted by Weight (excluding
Weight==0). A RewardNum==0 outcome skips both the grant and the
rewards[] emission. Empty WinCount catalogs emit empty arrays.

Existing seed entries preserve deterministic behavior by living in
single-option groups (each with weight 1). Future seasons can expand
groups to multi-option for true randomized rewards (e.g. 200-280
rupies).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/Migrations/20260531174101_AddArenaTwoPickRewardGroupAndWeight.cs

@@ -0,0 +1,60 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace SVSim.Database.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddArenaTwoPickRewardGroupAndWeight : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_ArenaTwoPickRewards_WinCount_RewardType_RewardId",
+                table: "ArenaTwoPickRewards");
+
+            migrationBuilder.AddColumn<int>(
+                name: "RewardGroup",
+                table: "ArenaTwoPickRewards",
+                type: "integer",
+                nullable: false,
+                defaultValue: 0);
+
+            migrationBuilder.AddColumn<int>(
+                name: "Weight",
+                table: "ArenaTwoPickRewards",
+                type: "integer",
+                nullable: false,
+                defaultValue: 0);
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ArenaTwoPickRewards_WinCount_RewardGroup_RewardType_RewardI~",
+                table: "ArenaTwoPickRewards",
+                columns: new[] { "WinCount", "RewardGroup", "RewardType", "RewardId", "RewardNum" },
+                unique: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropIndex(
+                name: "IX_ArenaTwoPickRewards_WinCount_RewardGroup_RewardType_RewardI~",
+                table: "ArenaTwoPickRewards");
+
+            migrationBuilder.DropColumn(
+                name: "RewardGroup",
+                table: "ArenaTwoPickRewards");
+
+            migrationBuilder.DropColumn(
+                name: "Weight",
+                table: "ArenaTwoPickRewards");
+
+            migrationBuilder.CreateIndex(
+                name: "IX_ArenaTwoPickRewards_WinCount_RewardType_RewardId",
+                table: "ArenaTwoPickRewards",
+                columns: new[] { "WinCount", "RewardType", "RewardId" },
+                unique: true);
+        }
+    }
+}