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/API.Tests.csproj

@@ -0,0 +1,29 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.2" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+    <PackageReference Include="Moq" Version="4.20.72" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" Version="9.0.0" />
+    <PackageReference Include="Microsoft.AspNetCore.Mvc.Testing" Version="9.0.0" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\API\API.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+</Project>