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/Services/WorkerClient.cs

@@ -625,7 +625,8 @@ public sealed record AppSettingsDto(
     List<string>? SessionSkills = null,
     List<ModelPresetDto>? ModelPresets = null,
     int UsageGateFiveHourPct = 80,
-    int UsageGateSevenDayPct = 90);
+    int UsageGateSevenDayPct = 90,
+    int MaxTurnsCeiling = 80);
 
 // Per-model run defaults (effort + turn budget) edited in Settings → General.
 public sealed record ModelPresetDto(string Model, string Effort, int MaxTurns);