feat(data): add usage gate thresholds and task run model column

Adds AppSettings.UsageGateFiveHourPct/UsageGateSevenDayPct (defaults
80/90, clamped 0..100 in UpdateAsync) and a nullable TaskRunEntity.Model
column, laying the data foundation for the usage monitor. No worker/UI
changes.

src/ClaudeDo.Data/Repositories/AppSettingsRepository.cs

@@ -65,6 +65,8 @@ public sealed class AppSettingsRepository
         row.StandupWeekday = updated.StandupWeekday;
         row.DailyPrepMaxTasks = updated.DailyPrepMaxTasks < 1 ? 1 : updated.DailyPrepMaxTasks;
         row.SessionSkills = string.IsNullOrWhiteSpace(updated.SessionSkills) ? null : updated.SessionSkills;
+        row.UsageGateFiveHourPct = Math.Clamp(updated.UsageGateFiveHourPct, 0, 100);
+        row.UsageGateSevenDayPct = Math.Clamp(updated.UsageGateSevenDayPct, 0, 100);
 
         await _context.SaveChangesAsync(ct);
     }