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/Modals/SettingsModalViewModel.cs

@@ -59,6 +59,7 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
                 General.DefaultClaudeInstructions = dto.DefaultClaudeInstructions ?? "";
                 General.DefaultModel = dto.DefaultModel ?? "sonnet";
                 General.DefaultMaxTurns = dto.DefaultMaxTurns;
+                General.MaxTurnsCeiling = dto.MaxTurnsCeiling;
                 General.DefaultPermissionMode = dto.DefaultPermissionMode ?? "auto";
                 General.MaxParallelExecutions = dto.MaxParallelExecutions;
                 General.UsageGateFiveHourPct = dto.UsageGateFiveHourPct;
@@ -80,7 +81,7 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
             await OnlineInbox.LoadAsync();
             await SessionSkills.LoadAsync();
             await General.LoadSessionSkillsAsync(_worker, dto?.SessionSkills);
-            General.LoadModelPresets(dto?.ModelPresets);
+            General.LoadModelPresets(dto?.ModelPresets, General.MaxTurnsCeiling);
         }
         finally { IsBusy = false; }
     }
@@ -113,7 +114,8 @@ public sealed partial class SettingsModalViewModel : ViewModelBase
                 General.SelectedSessionSkillNames(),
                 General.ModelPresetDtos(),
                 General.UsageGateFiveHourPct,
-                General.UsageGateSevenDayPct);
+                General.UsageGateSevenDayPct,
+                General.MaxTurnsCeiling);
             await _worker.UpdateAppSettingsAsync(dto);
             await Prime.SaveAsync();
             await OnlineInbox.SaveAsync();