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.

tests/ClaudeDo.Worker.Tests/Runner/ModelResolutionWireTests.cs

@@ -87,9 +87,7 @@ public sealed class ModelResolutionWireTests : IDisposable
         using (var ctx = _db.CreateContext())
         {
             var settingsRepo = new AppSettingsRepository(ctx);
-            var settings = await settingsRepo.GetAsync();
-            settings.DefaultMaxTurns = 100;
-            await ctx.SaveChangesAsync();
+            await settingsRepo.UpdateAsync(new AppSettingsEntity { DefaultMaxTurns = 50 });
         }
 
         Exception? thrown = null;
@@ -106,6 +104,6 @@ public sealed class ModelResolutionWireTests : IDisposable
         Assert.Null(thrown);
         var args = getArgs().ToList();
         Assert.Contains("--max-turns", args);
-        Assert.Equal("100", args[args.IndexOf("--max-turns") + 1]);
+        Assert.Equal("50", args[args.IndexOf("--max-turns") + 1]);
     }
 }