feat(config): MatchingConfig section with AI-fallback threshold

Adds a [ConfigSection("Matching")] POCO carrying the
RankBattleAiFallbackThresholdSeconds tunable (default 15). Auto-picked
up by EnsureSeedDataAsync's reflection-based ConfigSection seeder.
Consumed by InProcessPairUp in a later task.

GameConfigurationJsonbTests.EnsureSeedData_writes_one_row_per_ConfigSection_with_ShippedDefaults_payload
updated to include the new Matching section in its expected keys.

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

SVSim.UnitTests/Database/Config/MatchingConfigTests.cs

@@ -0,0 +1,34 @@
+using System.Linq;
+using NUnit.Framework;
+using SVSim.Database.Models.Config;
+
+namespace SVSim.UnitTests.Database.Config;
+
+[TestFixture]
+public class MatchingConfigTests
+{
+    [Test]
+    public void Default_threshold_is_15_seconds()
+    {
+        var cfg = new MatchingConfig();
+        Assert.That(cfg.RankBattleAiFallbackThresholdSeconds, Is.EqualTo(15));
+    }
+
+    [Test]
+    public void ShippedDefaults_returns_a_new_instance_with_default_threshold()
+    {
+        var cfg = MatchingConfig.ShippedDefaults();
+        Assert.That(cfg.RankBattleAiFallbackThresholdSeconds, Is.EqualTo(15));
+    }
+
+    [Test]
+    public void Has_ConfigSection_attribute_with_name_Matching()
+    {
+        var attr = typeof(MatchingConfig)
+            .GetCustomAttributes(typeof(ConfigSectionAttribute), false)
+            .Cast<ConfigSectionAttribute>()
+            .FirstOrDefault();
+        Assert.That(attr, Is.Not.Null);
+        Assert.That(attr!.Name, Is.EqualTo("Matching"));
+    }
+}