feat(arena-colosseum): lobby + constructed entry (phase 1)

Closes 5 of arena-colosseum's 16 spec shapes (1/16 → 6/16). Lobby reads
(/top, /get_fee_info, /event_info) render an empty "no event scheduled"
payload by default; /entry + /register_deck activate via admin-flipped
ColosseumSeason + ColosseumRounds config sections.

* Schema: ViewerArenaColosseumRun standalone table (unique on ViewerId)
  with jsonb run-state columns mirroring ViewerArenaTwoPickRun.
* Config sections: ColosseumSeasonConfig (event-level), ColosseumRoundsConfig
  (the 3-round bracket). Empty defaults — IsColosseumPeriod=false.
* Migration AddArenaColosseumRun (DDL only; seed rows come from the section
  ShippedDefaults via EnsureSeedDataAsync).
* DTOs: ColosseumLobbyInfo (round-level, /top + /get_fee_info), ColosseumEventInfo
  (event-level, /event_info), ColosseumOwnStatus (shared status block),
  ColosseumEntryRef, ColosseumFeeList, ColosseumUserDeck, ColosseumBattleResults,
  ColosseumRoundDetail + ColosseumGroupRow. EventInfoResponse uses explicit
  [JsonPropertyName("1"|"2"|"3")] for the string-keyed rounds shape per spec.
* Controller: ArenaColosseumController replaces the 1-action stub with
  the five lifecycle endpoints. /top emits leader_skin_id even when 0
  (project_wire_null_policy override).
* Tests: 11 controller-level tests + 6 config round-trip tests covering
  empty-season payloads, run-seeded /top round-trip, crystal/rupy entry
  debits, now_round_id mismatch rejection, deck_no_list round-trip + reject.

SVSim.UnitTests/Database/Config/ColosseumRoundsConfigTests.cs

@@ -0,0 +1,46 @@
+using System.Linq;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using NUnit.Framework;
+using SVSim.Database;
+using SVSim.Database.Models.Config;
+using SVSim.EmulatedEntrypoint.Services;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Database.Config;
+
+[TestFixture]
+public class ColosseumRoundsConfigTests
+{
+    [Test]
+    public void ShippedDefaults_emits_empty_rounds()
+    {
+        var cfg = ColosseumRoundsConfig.ShippedDefaults();
+        Assert.That(cfg.Rounds, Is.Empty,
+            "default ship state has no rounds — /event_info renders a benign empty payload");
+    }
+
+    [Test]
+    public void Has_ConfigSection_attribute_with_name_ColosseumRounds()
+    {
+        var attr = typeof(ColosseumRoundsConfig)
+            .GetCustomAttributes(typeof(ConfigSectionAttribute), false)
+            .Cast<ConfigSectionAttribute>()
+            .FirstOrDefault();
+        Assert.That(attr, Is.Not.Null);
+        Assert.That(attr!.Name, Is.EqualTo("ColosseumRounds"));
+    }
+
+    [Test]
+    public void Get_through_GameConfigService_round_trips_shipped_defaults()
+    {
+        using var factory = new SVSimTestFactory();
+        using var scope = factory.Services.CreateScope();
+        var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+        var svc = new GameConfigService(db, new ConfigurationBuilder().Build());
+
+        var cfg = svc.Get<ColosseumRoundsConfig>();
+
+        Assert.That(cfg.Rounds, Is.Empty);
+    }
+}

SVSim.UnitTests/Database/Config/ColosseumSeasonConfigTests.cs

@@ -0,0 +1,47 @@
+using System.Linq;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using NUnit.Framework;
+using SVSim.Database;
+using SVSim.Database.Models.Config;
+using SVSim.EmulatedEntrypoint.Services;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Database.Config;
+
+[TestFixture]
+public class ColosseumSeasonConfigTests
+{
+    [Test]
+    public void ShippedDefaults_emits_no_period()
+    {
+        var cfg = ColosseumSeasonConfig.ShippedDefaults();
+        Assert.That(cfg.IsColosseumPeriod, Is.False,
+            "default ship state is no event scheduled — lobby reads must render the empty payload");
+    }
+
+    [Test]
+    public void Has_ConfigSection_attribute_with_name_ColosseumSeason()
+    {
+        var attr = typeof(ColosseumSeasonConfig)
+            .GetCustomAttributes(typeof(ConfigSectionAttribute), false)
+            .Cast<ConfigSectionAttribute>()
+            .FirstOrDefault();
+        Assert.That(attr, Is.Not.Null);
+        Assert.That(attr!.Name, Is.EqualTo("ColosseumSeason"));
+    }
+
+    [Test]
+    public void Get_through_GameConfigService_round_trips_shipped_defaults()
+    {
+        using var factory = new SVSimTestFactory();
+        using var scope = factory.Services.CreateScope();
+        var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+        var svc = new GameConfigService(db, new ConfigurationBuilder().Build());
+
+        var cfg = svc.Get<ColosseumSeasonConfig>();
+
+        Assert.That(cfg.IsColosseumPeriod, Is.False);
+        Assert.That(cfg.PoolCardSetIds, Is.Empty);
+    }
+}