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.Database/Models/Config/MatchingConfig.cs

@@ -0,0 +1,20 @@
+namespace SVSim.Database.Models.Config;
+
+/// <summary>
+/// Tunables for the in-process pair-up matching service. Today: just the AI-fallback
+/// threshold for rank-battle modes. The full matching-queue API is a separate spec;
+/// this config section lives alongside the placeholder.
+/// </summary>
+[ConfigSection("Matching")]
+public class MatchingConfig
+{
+    /// <summary>
+    /// How long (seconds) a viewer must have been parked in a PvpFirstThenAiFallback
+    /// queue before their next /do_matching poll resolves to an AI battle.
+    /// Defaults to 15 — matches the prod 4s pre-AIBattleStart pause plus a comfortable
+    /// polling cycle.
+    /// </summary>
+    public int RankBattleAiFallbackThresholdSeconds { get; set; } = 15;
+
+    public static MatchingConfig ShippedDefaults() => new();
+}