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.Worker/Runner/TaskRunner.cs

@@ -522,12 +522,21 @@ public sealed class TaskRunner
         var model = task.Model ?? listConfig?.Model ?? global.DefaultModel;
         var preset = Data.Models.ModelPresets.For(global.ModelPresets, model, global.DefaultMaxTurns);
 
+        var requestedMaxTurns = task.MaxTurns ?? listConfig?.MaxTurns ?? preset.MaxTurns;
+        var maxTurns = ResolveMaxTurns(task.MaxTurns, listConfig?.MaxTurns, preset.MaxTurns, global.MaxTurnsCeiling);
+        if (maxTurns < requestedMaxTurns)
+        {
+            _logger.LogWarning(
+                "Task {TaskId}: max turns clamped to ceiling (requested={Requested}, effective={Effective}, ceiling={Ceiling})",
+                task.Id, requestedMaxTurns, maxTurns, global.MaxTurnsCeiling);
+        }
+
         return new ClaudeRunConfig(
             Model: model,
             SystemPrompt: string.IsNullOrWhiteSpace(instructions) ? null : instructions,
             AgentPath: task.AgentPath ?? listConfig?.AgentPath,
             ResumeSessionId: resumeSessionId,
-            MaxTurns: ResolveMaxTurns(task.MaxTurns, listConfig?.MaxTurns, preset.MaxTurns),
+            MaxTurns: maxTurns,
             PermissionMode: global.DefaultPermissionMode,
             SkillNames: skillNames,
             Effort: preset.Effort);
@@ -586,8 +595,8 @@ public sealed class TaskRunner
         return names;
     }
 
-    internal static int? ResolveMaxTurns(int? taskTurns, int? listTurns, int globalDefault)
-        => taskTurns ?? listTurns ?? globalDefault;
+    internal static int? ResolveMaxTurns(int? taskTurns, int? listTurns, int globalDefault, int ceiling)
+        => Math.Min(taskTurns ?? listTurns ?? globalDefault, ceiling);
 
     public static string MergeInstructions(params string?[] parts)
     {