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.Localization/locales/de.json

@@ -103,6 +103,7 @@
       "overrideBadge": "überschrieben",
       "resetToInherited": "Auf geerbt zurücksetzen"
     },
+    "turnsCeilingHint": "Läufe sind auf {0} Durchläufe gedeckelt — dieser Wert wird geklemmt.",
     "agentEditor": {
       "model": "Modell",
       "maxTurns": "Max. Durchläufe",