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/Services/AgeGroupServiceTests.cs

@@ -0,0 +1,169 @@
+using API.Database;
+using API.Models.Internal.Altersgruppen;
+using API.Repository.AgeGroupRepo;
+using Microsoft.EntityFrameworkCore;
+
+namespace API.Tests.Services;
+
+public class AgeGroupServiceTests : IDisposable
+{
+    private readonly ApplicationDbContext _context;
+    private readonly AgeGroupService _service;
+
+    public AgeGroupServiceTests()
+    {
+        var options = new DbContextOptionsBuilder<ApplicationDbContext>()
+            .UseInMemoryDatabase(databaseName: Guid.NewGuid().ToString())
+            .Options;
+
+        _context = new ApplicationDbContext(options);
+        _service = new AgeGroupService(_context);
+    }
+
+    public void Dispose()
+    {
+        _context.Database.EnsureDeleted();
+        _context.Dispose();
+    }
+
+    [Fact]
+    public async Task GetAllAsync_EmptyDatabase_ReturnsEmptyList()
+    {
+        // Act
+        var result = await _service.GetAllAsync();
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Empty(result);
+    }
+
+    [Fact]
+    public async Task GetAllAsync_WithData_ReturnsAllGroups()
+    {
+        // Arrange
+        var group1 = new AltersGruppe { Name = "U10", StartingAge = 8, EndingAge = 10 };
+        var group2 = new AltersGruppe { Name = "U12", StartingAge = 10, EndingAge = 12 };
+        await _context.Altersgruppen.AddRangeAsync(group1, group2);
+        await _context.SaveChangesAsync();
+
+        // Act
+        var result = await _service.GetAllAsync();
+
+        // Assert
+        Assert.Equal(2, result.Count);
+        Assert.Contains(result, g => g.Name == "U10");
+        Assert.Contains(result, g => g.Name == "U12");
+    }
+
+    [Fact]
+    public async Task GetAsync_ExistingId_ReturnsGroup()
+    {
+        // Arrange
+        var group = new AltersGruppe { Name = "U14", StartingAge = 12, EndingAge = 14 };
+        await _context.Altersgruppen.AddAsync(group);
+        await _context.SaveChangesAsync();
+
+        // Act
+        var result = await _service.GetAsync(group.Id);
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Equal("U14", result.Name);
+        Assert.Equal(12, result.StartingAge);
+        Assert.Equal(14, result.EndingAge);
+    }
+
+    [Fact]
+    public async Task GetAsync_NonExistingId_ReturnsNull()
+    {
+        // Act
+        var result = await _service.GetAsync("nonexistent-id");
+
+        // Assert
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public async Task CreateAsync_ValidGroup_AddsToDatabase()
+    {
+        // Arrange
+        var newGroup = new AltersGruppe { Name = "U16", StartingAge = 14, EndingAge = 16 };
+
+        // Act
+        var result = await _service.CreateAsync(newGroup);
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Equal("U16", result.Name);
+
+        var dbGroup = await _context.Altersgruppen.FindAsync(result.Id);
+        Assert.NotNull(dbGroup);
+        Assert.Equal("U16", dbGroup.Name);
+    }
+
+    [Fact]
+    public async Task UpdateAsync_ExistingGroup_UpdatesProperties()
+    {
+        // Arrange
+        var group = new AltersGruppe { Name = "U18", StartingAge = 16, EndingAge = 18 };
+        await _context.Altersgruppen.AddAsync(group);
+        await _context.SaveChangesAsync();
+
+        var updatedGroup = new AltersGruppe { Name = "U18 Updated", StartingAge = 15, EndingAge = 18 };
+
+        // Act
+        var result = await _service.UpdateAsync(group.Id, updatedGroup);
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Equal("U18 Updated", result.Name);
+        Assert.Equal(15, result.StartingAge);
+        Assert.Equal(18, result.EndingAge);
+
+        var dbGroup = await _context.Altersgruppen.FindAsync(group.Id);
+        Assert.NotNull(dbGroup);
+        Assert.Equal("U18 Updated", dbGroup.Name);
+    }
+
+    [Fact]
+    public async Task UpdateAsync_NonExistingGroup_ReturnsNull()
+    {
+        // Arrange
+        var updatedGroup = new AltersGruppe { Name = "U18 Updated", StartingAge = 15, EndingAge = 18 };
+
+        // Act
+        var result = await _service.UpdateAsync("nonexistent-id", updatedGroup);
+
+        // Assert
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public async Task DeleteAsync_ExistingGroup_RemovesFromDatabase()
+    {
+        // Arrange
+        var group = new AltersGruppe { Name = "U20", StartingAge = 18, EndingAge = 20 };
+        await _context.Altersgruppen.AddAsync(group);
+        await _context.SaveChangesAsync();
+
+        // Act
+        var result = await _service.DeleteAsync(group.Id);
+
+        // Assert
+        Assert.NotNull(result);
+        Assert.Equal("U20", result.Name);
+
+        var dbGroup = await _context.Altersgruppen.FindAsync(group.Id);
+        Assert.Null(dbGroup);
+    }
+
+    [Fact]
+    public async Task DeleteAsync_NonExistingGroup_ReturnsNull()
+    {
+        // Act
+        var result = await _service.DeleteAsync("nonexistent-id");
+
+        // Assert
+        Assert.Null(result);
+    }
+}