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/Services/TransactionStatisticsServiceTests.cs

@@ -0,0 +1,188 @@
+using MoneyMap.Models;
+using MoneyMap.Services;
+using MoneyMap.Tests.TestHelpers;
+using Xunit;
+
+namespace MoneyMap.Tests.Services;
+
+public class TransactionStatisticsServiceTests
+{
+    [Fact]
+    public async Task CalculateStatsAsync_ReturnsCorrectStatistics()
+    {
+        // Arrange
+        using var context = DbContextHelper.CreateInMemoryContext();
+        var service = new TransactionStatisticsService(context);
+
+        var account = new Account
+        {
+            Id = 1,
+            Institution = "Test Bank",
+            AccountType = AccountType.Checking,
+            Last4 = "1234",
+            Owner = "Test Owner"
+        };
+        context.Accounts.Add(account);
+
+        var debit1 = new Transaction
+        {
+            Date = DateTime.Now,
+            Amount = -50.00m,
+            Name = "Store A",
+            Memo = "Purchase",
+            AccountId = 1
+        };
+        var debit2 = new Transaction
+        {
+            Date = DateTime.Now,
+            Amount = -30.00m,
+            Name = "Store B",
+            Memo = "Purchase",
+            AccountId = 1
+        };
+        var credit = new Transaction
+        {
+            Date = DateTime.Now,
+            Amount = 100.00m,
+            Name = "Deposit",
+            Memo = "Paycheck",
+            AccountId = 1
+        };
+        context.Transactions.AddRange(debit1, debit2, credit);
+        await context.SaveChangesAsync();
+
+        var query = context.Transactions.AsQueryable();
+
+        // Act
+        var result = await service.CalculateStatsAsync(query);
+
+        // Assert
+        Assert.Equal(3, result.Count);
+        Assert.Equal(-80.00m, result.TotalDebits);
+        Assert.Equal(100.00m, result.TotalCredits);
+        Assert.Equal(20.00m, result.NetAmount);
+    }
+
+    [Fact]
+    public async Task GetCategorizationStatsAsync_ReturnsCorrectCounts()
+    {
+        // Arrange
+        using var context = DbContextHelper.CreateInMemoryContext();
+        var service = new TransactionStatisticsService(context);
+
+        var account = new Account
+        {
+            Id = 1,
+            Institution = "Test Bank",
+            AccountType = AccountType.Checking,
+            Last4 = "1234",
+            Owner = "Test Owner"
+        };
+        context.Accounts.Add(account);
+
+        var categorized1 = new Transaction
+        {
+            Date = DateTime.Now,
+            Amount = -50.00m,
+            Name = "Test",
+            Memo = "Test",
+            AccountId = 1,
+            Category = "Groceries"
+        };
+        var categorized2 = new Transaction
+        {
+            Date = DateTime.Now,
+            Amount = -30.00m,
+            Name = "Test",
+            Memo = "Test",
+            AccountId = 1,
+            Category = "Gas"
+        };
+        var uncategorized = new Transaction
+        {
+            Date = DateTime.Now,
+            Amount = -20.00m,
+            Name = "Test",
+            Memo = "Test",
+            AccountId = 1,
+            Category = ""
+        };
+        context.Transactions.AddRange(categorized1, categorized2, uncategorized);
+        await context.SaveChangesAsync();
+
+        // Act
+        var result = await service.GetCategorizationStatsAsync();
+
+        // Assert
+        Assert.Equal(3, result.TotalTransactions);
+        Assert.Equal(2, result.Categorized);
+        Assert.Equal(1, result.Uncategorized);
+    }
+
+    [Fact]
+    public async Task GetCardStatsForAccountAsync_ReturnsStatsForLinkedCards()
+    {
+        // Arrange
+        using var context = DbContextHelper.CreateInMemoryContext();
+        var service = new TransactionStatisticsService(context);
+
+        var account = new Account
+        {
+            Id = 1,
+            Institution = "Test Bank",
+            AccountType = AccountType.Checking,
+            Last4 = "1234",
+            Owner = "Test Owner"
+        };
+        context.Accounts.Add(account);
+
+        var card1 = new Card
+        {
+            Id = 1,
+            AccountId = 1,
+            Issuer = "VISA",
+            Last4 = "1111",
+            Owner = "Test"
+        };
+        var card2 = new Card
+        {
+            Id = 2,
+            AccountId = 1,
+            Issuer = "Mastercard",
+            Last4 = "2222",
+            Owner = "Test"
+        };
+        context.Cards.AddRange(card1, card2);
+
+        var transaction1 = new Transaction
+        {
+            Date = DateTime.Now,
+            Amount = -50.00m,
+            Name = "Test",
+            Memo = "Test",
+            AccountId = 1,
+            CardId = 1
+        };
+        var transaction2 = new Transaction
+        {
+            Date = DateTime.Now,
+            Amount = -30.00m,
+            Name = "Test",
+            Memo = "Test",
+            AccountId = 1,
+            CardId = 1
+        };
+        context.Transactions.AddRange(transaction1, transaction2);
+        await context.SaveChangesAsync();
+
+        // Act
+        var result = await service.GetCardStatsForAccountAsync(1);
+
+        // Assert
+        Assert.Equal(2, result.Count);
+        var card1Stats = result.First(c => c.Card.Id == 1);
+        Assert.Equal(2, card1Stats.TransactionCount);
+        var card2Stats = result.First(c => c.Card.Id == 2);
+        Assert.Equal(0, card2Stats.TransactionCount);
+    }
+}