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.

src/ClaudeDo.Ui/ViewModels/Agent/AgentConfigEditorViewModel.cs

@@ -43,6 +43,7 @@ public sealed partial class AgentConfigEditorViewModel : ViewModelBase, IDisposa
     [ObservableProperty] private string _modelInheritedHint = "";
     [ObservableProperty] private string _turnsBadge = "";
     [ObservableProperty] private string _turnsInheritedHint = "";
+    [ObservableProperty] private string _turnsCeilingHint = "";
     [ObservableProperty] private string _agentBadge = "";
     [ObservableProperty] private string _effectiveSystemPromptHint = "";
 
@@ -50,6 +51,7 @@ public sealed partial class AgentConfigEditorViewModel : ViewModelBase, IDisposa
     // The global max-turns default is per-model (Settings -> General), so it moves with whichever
     // model actually ends up in effect here.
     private IReadOnlyList<ModelPreset> _presets = ModelPresets.Defaults;
+    private int _maxTurnsCeiling = 80;
     private string EffectiveModel => Model ?? _listModel ?? _globalModel;
     private int GlobalMaxTurns => ModelPresets.For(_presets, EffectiveModel).MaxTurns;
     private string? _listModel;     // Task scope only
@@ -145,6 +147,9 @@ public sealed partial class AgentConfigEditorViewModel : ViewModelBase, IDisposa
             : InheritanceResolver.ResolveList(own, GlobalMaxTurns.ToString());
         TurnsInheritedHint = value;
         TurnsBadge = BadgeFor(source, MaxTurns is not null);
+        TurnsCeilingHint = MaxTurns is decimal t && (int)t > _maxTurnsCeiling
+            ? Loc.T("settings.turnsCeilingHint", _maxTurnsCeiling)
+            : "";
     }
 
     private void RecomputeAgentBadge()
@@ -301,6 +306,7 @@ public sealed partial class AgentConfigEditorViewModel : ViewModelBase, IDisposa
         _presets = app?.ModelPresets is { Count: > 0 } rows
             ? rows.Select(r => new ModelPreset(r.Model, r.Effort, r.MaxTurns)).ToList()
             : ModelPresets.Defaults;
+        _maxTurnsCeiling = app?.MaxTurnsCeiling ?? 80;
     }
 
     private void ApplyConfig(string? model, int? maxTurns, string? systemPrompt, string? agentPath)