fix(worker): filter the transliterated 'fuer' stopword, not the untransliterated 'fur'

NormalizeTitleWords transliterates umlauts before tokenizing ('für' ->
'fuer'), but TitleStopWords listed 'fur' -- a token the pipeline can
never produce -- so 'für' was never filtered as filler and could push
unrelated titles past the near-duplicate threshold.

src/ClaudeDo.Worker/External/ExternalMcpService.cs

@@ -329,7 +329,7 @@ public sealed class ExternalMcpService
     // plus the recurring "mcp"/"task(s)"/"cleanup" nouns called out in the task write-up).
     private static readonly HashSet<string> TitleStopWords = new(StringComparer.Ordinal)
     {
-        "der", "die", "das", "und", "von", "auf", "mit", "fur", "ein", "eine",
+        "der", "die", "das", "und", "von", "auf", "mit", "fuer", "ein", "eine",
         "ist", "sind", "oder", "nicht", "mcp", "task", "tasks", "cleanup",
     };
 

tests/ClaudeDo.Worker.Tests/External/ExternalMcpServiceTests.cs

@@ -1829,6 +1829,24 @@ public sealed class ExternalMcpServiceTests : IDisposable
         Assert.Empty(result.PossibleDuplicates);
     }
 
+    [Fact]
+    public async Task AddTask_TitlesSharingOnlyOneWordPlusTheGermanStopwordFuer_AreNotReportedAsDuplicates()
+    {
+        // Regression for the "fur" (never-transliterated) stopword typo: NormalizeTitleWords
+        // transliterates "für" -> "fuer" before the stopword check, so a stopword list containing
+        // "fur" can never filter it. Before the fix these two titles shared "report" and "fuer"
+        // (2 words, 50% of the shorter title) and cleared the duplicate threshold undeservedly;
+        // with "fuer" correctly filtered as filler only "report" remains shared (1 word).
+        var listId = await SeedListAsync();
+        await SeedTaskAsync(listId, "Neuer Report für Bestand", TaskStatus.Idle);
+        var sut = NewService();
+
+        var result = await sut.AddTask(
+            listId, "Anderer Report für Kunden", cancellationToken: CancellationToken.None);
+
+        Assert.Empty(result.PossibleDuplicates);
+    }
+
     [Fact]
     public async Task AddTask_MoreThanThreeMatches_CapsPossibleDuplicatesAtThree()
     {