Add comprehensive unit tests for backend services and controllers

- Add API.Tests xUnit project with Moq and EF Core InMemory
- Add AgeGroupService tests (CRUD operations, edge cases)
- Add RegistrationKeyService tests (CRUD + DeleteOldRegistrationKeys)
- Add AgeGroupController tests (all endpoints with mocked service)
- Add PlunkEmailSender tests (HTTP client mocking, payload verification)
- Add Mapper tests (AltersgruppeMapper, RegistrationKeyMapper)

https://claude.ai/code/session_01Kv7Mp2c9FKsHEgQe4BoftH

API.Tests/Mappers/AltersgruppeMapperTests.cs

@@ -0,0 +1,118 @@
+using API.Models.Internal.Altersgruppen;
+
+namespace API.Tests.Mappers;
+
+public class AltersgruppeMapperTests
+{
+    [Fact]
+    public void ToInternalFromIngoing_MapsAllProperties()
+    {
+        // Arrange
+        var ingoing = new AltersGruppeIngoing
+        {
+            Name = "U14",
+            StartingAge = 12,
+            EndingAge = 14
+        };
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Equal("U14", result.Name);
+        Assert.Equal(12, result.StartingAge);
+        Assert.Equal(14, result.EndingAge);
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_GeneratesNewId()
+    {
+        // Arrange
+        var ingoing = new AltersGruppeIngoing
+        {
+            Name = "U16",
+            StartingAge = 14,
+            EndingAge = 16
+        };
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        Assert.NotNull(result.Id);
+        Assert.NotEmpty(result.Id);
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_GeneratesUniqueIds()
+    {
+        // Arrange
+        var ingoing1 = new AltersGruppeIngoing { Name = "U10", StartingAge = 8, EndingAge = 10 };
+        var ingoing2 = new AltersGruppeIngoing { Name = "U12", StartingAge = 10, EndingAge = 12 };
+
+        // Act
+        var result1 = ingoing1.ToInternalFromIngoing();
+        var result2 = ingoing2.ToInternalFromIngoing();
+
+        // Assert
+        Assert.NotEqual(result1.Id, result2.Id);
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_WithZeroAges_MapsCorrectly()
+    {
+        // Arrange
+        var ingoing = new AltersGruppeIngoing
+        {
+            Name = "Beginners",
+            StartingAge = 0,
+            EndingAge = 0
+        };
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        Assert.Equal(0, result.StartingAge);
+        Assert.Equal(0, result.EndingAge);
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_WithNegativeAges_MapsCorrectly()
+    {
+        // Arrange - testing edge case, though business logic might not allow this
+        var ingoing = new AltersGruppeIngoing
+        {
+            Name = "Invalid",
+            StartingAge = -1,
+            EndingAge = -5
+        };
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        Assert.Equal(-1, result.StartingAge);
+        Assert.Equal(-5, result.EndingAge);
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_WithLargeAges_MapsCorrectly()
+    {
+        // Arrange
+        var ingoing = new AltersGruppeIngoing
+        {
+            Name = "Seniors",
+            StartingAge = 60,
+            EndingAge = 100
+        };
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        Assert.Equal(60, result.StartingAge);
+        Assert.Equal(100, result.EndingAge);
+    }
+}