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/RegistrationKeyMapperTests.cs

@@ -0,0 +1,123 @@
+using API.Models.Internal.User;
+
+namespace API.Tests.Mappers;
+
+public class RegistrationKeyMapperTests
+{
+    [Fact]
+    public void ToInternalFromIngoing_MapsLinkedRole()
+    {
+        // Arrange
+        var ingoing = new RegistrationKeyIngoing
+        {
+            LinkedRole = "Admin"
+        };
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Equal("Admin", result.LinkedRole);
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_NullLinkedRole_MapsCorrectly()
+    {
+        // Arrange
+        var ingoing = new RegistrationKeyIngoing
+        {
+            LinkedRole = null
+        };
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Null(result.LinkedRole);
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_GeneratesNewId()
+    {
+        // Arrange
+        var ingoing = new RegistrationKeyIngoing
+        {
+            LinkedRole = "User"
+        };
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        Assert.NotNull(result.Id);
+        Assert.NotEmpty(result.Id);
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_GeneratesUniqueIds()
+    {
+        // Arrange
+        var ingoing1 = new RegistrationKeyIngoing { LinkedRole = "Admin" };
+        var ingoing2 = new RegistrationKeyIngoing { LinkedRole = "User" };
+
+        // Act
+        var result1 = ingoing1.ToInternalFromIngoing();
+        var result2 = ingoing2.ToInternalFromIngoing();
+
+        // Assert
+        Assert.NotEqual(result1.Id, result2.Id);
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_SetsCreatedDate()
+    {
+        // Arrange
+        var ingoing = new RegistrationKeyIngoing
+        {
+            LinkedRole = "Moderator"
+        };
+        var beforeCreation = DateTime.UtcNow;
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        var afterCreation = DateTime.UtcNow;
+        Assert.True(result.Created >= beforeCreation.AddSeconds(-1));
+        Assert.True(result.Created <= afterCreation.AddSeconds(1));
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_EmptyLinkedRole_MapsCorrectly()
+    {
+        // Arrange
+        var ingoing = new RegistrationKeyIngoing
+        {
+            LinkedRole = ""
+        };
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        Assert.Equal("", result.LinkedRole);
+    }
+
+    [Fact]
+    public void ToInternalFromIngoing_WhitespaceLinkedRole_MapsCorrectly()
+    {
+        // Arrange
+        var ingoing = new RegistrationKeyIngoing
+        {
+            LinkedRole = "   "
+        };
+
+        // Act
+        var result = ingoing.ToInternalFromIngoing();
+
+        // Assert
+        Assert.Equal("   ", result.LinkedRole);
+    }
+}