feat(worker): clamp max-turns to a configurable ceiling

Runaway sessions were the single biggest cost driver: model_presets was
never persisted (stayed code-only), default_max_turns shipped at 100, and
ResolveMaxTurns had no upper bound, so a task/list override could run
hundreds of turns unchecked.

- TaskRunner.ResolveMaxTurns now clamps the resolved value to
  AppSettings.MaxTurnsCeiling (new column, default 80) and logs a warning
  with task id / requested / effective value when it clamps.
- default_max_turns default lowered from 100 to 40 (entity, EF config,
  and the seeded row via the new AddMaxTurnsCeiling migration).
- AppSettingsRepository.GetAsync backfills model_presets with the
  shipping defaults on first read instead of leaving the column null.
- Settings > General's per-model preset table and the task/list agent
  editor now show a hint when a set max-turns value exceeds the ceiling.

tests/ClaudeDo.Worker.Tests/Repositories/AppSettingsRepositoryTests.cs

@@ -20,13 +20,49 @@ public class AppSettingsRepositoryTests : IDisposable
 
         Assert.Equal(AppSettingsEntity.SingletonId, row.Id);
         Assert.Equal("sonnet", row.DefaultModel);
-        Assert.Equal(100, row.DefaultMaxTurns);
+        Assert.Equal(40, row.DefaultMaxTurns);
+        Assert.Equal(80, row.MaxTurnsCeiling);
         Assert.Equal("auto", row.DefaultPermissionMode);
         Assert.Equal("sibling", row.WorktreeStrategy);
         Assert.Null(row.CentralWorktreeRoot);
         Assert.False(row.WorktreeAutoCleanupEnabled);
     }
 
+    [Fact]
+    public async Task UpdateAsync_Persists_MaxTurnsCeiling()
+    {
+        using (var ctx = _db.CreateContext())
+        {
+            var repo = new AppSettingsRepository(ctx);
+            await repo.UpdateAsync(new AppSettingsEntity { MaxTurnsCeiling = 60 });
+        }
+
+        using var readCtx = _db.CreateContext();
+        var row = await new AppSettingsRepository(readCtx).GetAsync();
+        Assert.Equal(60, row.MaxTurnsCeiling);
+    }
+
+    [Fact]
+    public async Task GetAsync_Backfills_Null_ModelPresets_With_Defaults()
+    {
+        using (var ctx = _db.CreateContext())
+        {
+            var repo = new AppSettingsRepository(ctx);
+            // Force a row to exist with model_presets left null (the pre-upgrade state).
+            await repo.UpdateAsync(new AppSettingsEntity());
+        }
+
+        using var readCtx = _db.CreateContext();
+        var row = await new AppSettingsRepository(readCtx).GetAsync();
+
+        Assert.NotNull(row.ModelPresets);
+        Assert.Equal(ModelPresets.SerializeDefaults(), row.ModelPresets);
+
+        using var rereadCtx = _db.CreateContext();
+        var reread = await new AppSettingsRepository(rereadCtx).GetAsync();
+        Assert.Equal(ModelPresets.SerializeDefaults(), reread.ModelPresets);
+    }
+
     [Fact]
     public async Task UpdateAsync_Persists_And_RoundTrips()
     {