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/Migrations/ClaudeDoDbContextModelSnapshot.cs

@@ -43,7 +43,7 @@ namespace ClaudeDo.Data.Migrations
                     b.Property<int>("DefaultMaxTurns")
                         .ValueGeneratedOnAdd()
                         .HasColumnType("INTEGER")
-                        .HasDefaultValue(30)
+                        .HasDefaultValue(40)
                         .HasColumnName("default_max_turns");
 
                     b.Property<string>("DefaultModel")
@@ -66,6 +66,12 @@ namespace ClaudeDo.Data.Migrations
                         .HasDefaultValue(1)
                         .HasColumnName("max_parallel_executions");
 
+                    b.Property<int>("MaxTurnsCeiling")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("INTEGER")
+                        .HasDefaultValue(80)
+                        .HasColumnName("max_turns_ceiling");
+
                     b.Property<string>("ModelPresets")
                         .HasColumnType("TEXT")
                         .HasColumnName("model_presets");
@@ -129,10 +135,11 @@ namespace ClaudeDo.Data.Migrations
                             Id = 1,
                             DailyPrepMaxTasks = 5,
                             DefaultClaudeInstructions = "",
-                            DefaultMaxTurns = 100,
+                            DefaultMaxTurns = 40,
                             DefaultModel = "sonnet",
                             DefaultPermissionMode = "auto",
                             MaxParallelExecutions = 1,
+                            MaxTurnsCeiling = 80,
                             StandupWeekday = 3,
                             UsageGateFiveHourPct = 80,
                             UsageGateSevenDayPct = 90,