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/SVSimDbContextModelSnapshot.cs

@@ -458,6 +458,9 @@ namespace SVSim.Database.Migrations
 
                     NpgsqlPropertyBuilderExtensions.UseIdentityByDefaultColumn(b.Property<long>("Id"));
 
+                    b.Property<int>("RewardGroup")
+                        .HasColumnType("integer");
+
                     b.Property<long>("RewardId")
                         .HasColumnType("bigint");
 
@@ -467,6 +470,9 @@ namespace SVSim.Database.Migrations
                     b.Property<int>("RewardType")
                         .HasColumnType("integer");
 
+                    b.Property<int>("Weight")
+                        .HasColumnType("integer");
+
                     b.Property<int>("WinCount")
                         .HasColumnType("integer");
 
@@ -474,7 +480,7 @@ namespace SVSim.Database.Migrations
 
                     b.HasIndex("WinCount");
 
-                    b.HasIndex("WinCount", "RewardType", "RewardId")
+                    b.HasIndex("WinCount", "RewardGroup", "RewardType", "RewardId", "RewardNum")
                         .IsUnique();
 
                     b.ToTable("ArenaTwoPickRewards");