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.Data/Models/AppSettingsEntity.cs

@@ -8,9 +8,13 @@ public sealed class AppSettingsEntity
 
     public string DefaultClaudeInstructions { get; set; } = string.Empty;
     public string DefaultModel { get; set; } = "sonnet";
-    public int DefaultMaxTurns { get; set; } = 100;
+    public int DefaultMaxTurns { get; set; } = 40;
     public string DefaultPermissionMode { get; set; } = "auto";
 
+    // Hard ceiling every resolved max-turns value (task/list/global) is clamped to before a run
+    // starts. Guards against runaway sessions regardless of what a task/list override requests.
+    public int MaxTurnsCeiling { get; set; } = 80;
+
     public int MaxParallelExecutions { get; set; } = 1;
 
     public string WorktreeStrategy { get; set; } = "sibling";