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/TestHelpers/DbContextHelper.cs

@@ -0,0 +1,20 @@
+using Microsoft.EntityFrameworkCore;
+using MoneyMap.Data;
+
+namespace MoneyMap.Tests.TestHelpers;
+
+public static class DbContextHelper
+{
+    /// <summary>
+    /// Creates an in-memory database context for testing.
+    /// Each call creates a unique database to ensure test isolation.
+    /// </summary>
+    public static MoneyMapContext CreateInMemoryContext()
+    {
+        var options = new DbContextOptionsBuilder<MoneyMapContext>()
+            .UseInMemoryDatabase(databaseName: Guid.NewGuid().ToString())
+            .Options;
+
+        return new MoneyMapContext(options);
+    }
+}