feat(worker): Toggle "Continue on session limit reset" für Usage-Limit-Abbrüche

Klassifiziert einen echten Usage-Limit-Abbruch als eigene FailureReason
"usage_limit" (TaskRunner.ClassifyFailureReason: nur bei terminal_reason
"api_error" plus einem Limit-Muster im gerenderten Fehlertext, nicht an
Status==Failed allein). Neuer Toggle AutoContinueOnUsageLimit (app_settings,
Default aus) unter Settings → General → "Usage limit stop":

- UsageLimitAutoContinueCoordinator feuert pro Task genau einmal ContinueTask
  über OverrideSlotService, sobald das 5h-Fenster (UsageState.Snapshot.FiveHour
  .ResetsAt) tatsächlich zurückgesetzt ist; ein persistenter Marker
  (TaskEntity.UsageLimitAutoContinuedAt) verhindert einen zweiten Anlauf bei
  einem erneuten Limit-Treffer.
- QueueService schedult zusätzlich einen exakten Wake-Timer auf den
  Reset-Zeitpunkt, statt nur auf den 30s-Backstop zu warten.
- Fail-open durchgängig: kein Snapshot/keine Reset-Zeit → kein Timer, kein
  Continue, kein Throw. Toggle aus ändert das heutige Verhalten nicht.

Migration AddUsageLimitAutoContinue fügt beide Spalten hinzu; die von
`dotnet ef migrations add` mitgescaffoldete leere UpdateData auf app_settings
(columns/values: []) erzeugte ungültiges SQL ("near WHERE") und wurde entfernt
— TaskNumberMigrationTests deckte das über den vollen Migrate()-Pfad auf.

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

@@ -99,6 +99,29 @@ public class FailureDiagnosisTests
     {
         Assert.Equal(expected, TaskRunner.ClassifyFailureReason(terminalReason));
     }
+
+    [Theory]
+    [InlineData("You've hit your session limit – resets 1pm (Europe/Berlin)")]
+    [InlineData("Claude usage limit reached, resets at 3pm")]
+    [InlineData("rate limited, please retry later")]
+    public void ClassifyFailureReason_ApiError_With_Limit_Text_Returns_UsageLimit(string messageText)
+    {
+        Assert.Equal("usage_limit", TaskRunner.ClassifyFailureReason("api_error", messageText));
+    }
+
+    [Fact]
+    public void ClassifyFailureReason_ApiError_With_Generic_Text_Stays_Error()
+    {
+        Assert.Equal("error", TaskRunner.ClassifyFailureReason("api_error", "Internal server error, please retry"));
+    }
+
+    [Theory]
+    [InlineData("max_turns")]
+    [InlineData("timeout")]
+    public void ClassifyFailureReason_Ignores_Limit_Text_For_NonApiError_Reasons(string terminalReason)
+    {
+        Assert.Equal(terminalReason, TaskRunner.ClassifyFailureReason(terminalReason, "usage limit reached"));
+    }
 }
 
 public sealed class FailureDiagnosisEndToEndTests : IDisposable
@@ -282,4 +305,32 @@ public sealed class FailureDiagnosisEndToEndTests : IDisposable
         Assert.Equal("error", task.FailureReason);
         Assert.NotEqual("max_turns", task.FailureReason);
     }
+
+    [Fact]
+    public async Task Usage_Limit_Run_Sets_FailureReason_UsageLimit()
+    {
+        var dbFactory = _db.CreateFactory();
+        using (var ctx = _db.CreateContext())
+        {
+            ctx.Lists.Add(new ListEntity { Id = "l1", Name = "L", WorkingDir = null, CreatedAt = DateTime.UtcNow });
+            ctx.Tasks.Add(new TaskEntity { Id = "t1", ListId = "l1", Title = "T",
+                Status = TaskStatus.Running, CreatedAt = DateTime.UtcNow });
+            await ctx.SaveChangesAsync();
+        }
+        var fake = new FakeClaudeProcess((_, _, _, _, _) => Task.FromResult(new RunResult
+        {
+            ExitCode = 1,
+            TerminalReason = "api_error",
+            ResultMarkdown = "You've hit your session limit – resets 1pm (Europe/Berlin)",
+        }));
+        var runner = MakeRunner(dbFactory, fake);
+
+        using (var ctx = _db.CreateContext())
+            await runner.RunAsync((await new TaskRepository(ctx).GetByIdAsync("t1"))!, "slot-1", default, alreadyClaimed: true);
+
+        using var verify = _db.CreateContext();
+        var task = await new TaskRepository(verify).GetByIdAsync("t1");
+        Assert.Equal(TaskStatus.Failed, task!.Status);
+        Assert.Equal("usage_limit", task.FailureReason);
+    }
 }