Test: add comprehensive unit test project for services

Add MoneyMap.Tests project with xUnit tests for all new services:

- AccountServiceTests: Account retrieval, stats, validation, deletion
- CardServiceTests: Card retrieval, stats, validation, deletion
- MerchantServiceTests: Merchant CRUD operations
- ReferenceDataServiceTests: Reference data retrieval
- TransactionServiceTests: Duplicate detection, retrieval, deletion
- TransactionStatisticsServiceTests: Statistics calculations
- ReceiptMatchingServiceTests: Receipt-to-transaction matching logic
- DbContextHelper: In-memory database context factory for test isolation

Uses xUnit, Moq, and EF Core InMemory database. Solution file updated to include test project.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

MoneyMap.Tests/MoneyMap.Tests.csproj

@@ -0,0 +1,27 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>net9.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" Version="6.0.2" />
+    <PackageReference Include="Microsoft.EntityFrameworkCore.InMemory" Version="9.0.10" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.12.0" />
+    <PackageReference Include="Moq" Version="4.20.72" />
+    <PackageReference Include="xunit" Version="2.9.2" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\MoneyMap\MoneyMap.csproj" />
+  </ItemGroup>
+
+</Project>