Perf: Fix N+1 queries in entity services

- CardService.GetAllCardsWithStatsAsync: Use single query with Select projection
- AccountService.GetAccountDetailsAsync: Use single query with Select projection
- TransactionStatisticsService: Calculate stats at DB level, fix N+1 in GetCardStatsForAccountAsync

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

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

MoneyMap/Services/AccountService.cs

@@ -85,23 +85,17 @@ public class AccountService : IAccountService
         if (account == null)
             return null;
 
-        // Get cards linked to this account
-        var cards = await _db.Cards
+        // Single query with projection to avoid N+1
+        var cardStats = await _db.Cards
             .Where(c => c.AccountId == id)
             .OrderBy(c => c.Owner)
             .ThenBy(c => c.Last4)
-            .ToListAsync();
-
-        var cardStats = new List<CardWithStats>();
-        foreach (var card in cards)
-        {
-            var transactionCount = await _db.Transactions.CountAsync(t => t.CardId == card.Id);
-            cardStats.Add(new CardWithStats
+            .Select(c => new CardWithStats
             {
-                Card = card,
-                TransactionCount = transactionCount
-            });
-        }
+                Card = c,
+                TransactionCount = c.Transactions.Count
+            })
+            .ToListAsync();
 
         // Get transaction count for this account
         var accountTransactionCount = await _db.Transactions.CountAsync(t => t.AccountId == id);

MoneyMap/Services/CardService.cs

@@ -55,26 +55,17 @@ public class CardService : ICardService
 
     public async Task<List<CardWithStats>> GetAllCardsWithStatsAsync()
     {
-        var cards = await _db.Cards
+        // Single query with projection to avoid N+1
+        return await _db.Cards
             .Include(c => c.Account)
             .OrderBy(c => c.Owner)
             .ThenBy(c => c.Last4)
-            .ToListAsync();
-
-        var cardStats = new List<CardWithStats>();
-
-        foreach (var card in cards)
-        {
-            var transactionCount = await _db.Transactions.CountAsync(t => t.CardId == card.Id);
-
-            cardStats.Add(new CardWithStats
+            .Select(c => new CardWithStats
             {
-                Card = card,
-                TransactionCount = transactionCount
-            });
-        }
-
-        return cardStats;
+                Card = c,
+                TransactionCount = c.Transactions.Count
+            })
+            .ToListAsync();
     }
 
     public async Task<DeleteValidationResult> CanDeleteCardAsync(int id)

MoneyMap/Services/TransactionStatisticsService.cs

@@ -36,15 +36,19 @@ public class TransactionStatisticsService : ITransactionStatisticsService
 
     public async Task<TransactionStats> CalculateStatsAsync(IQueryable<Transaction> query)
     {
-        var allFilteredTransactions = await query.ToListAsync();
+        // Calculate stats at database level instead of loading all transactions into memory
+        var stats = await query
+            .GroupBy(_ => 1) // Group all into one group to aggregate
+            .Select(g => new TransactionStats
+            {
+                Count = g.Count(),
+                TotalDebits = g.Where(t => t.Amount < 0).Sum(t => t.Amount),
+                TotalCredits = g.Where(t => t.Amount > 0).Sum(t => t.Amount),
+                NetAmount = g.Sum(t => t.Amount)
+            })
+            .FirstOrDefaultAsync();
 
-        return new TransactionStats
-        {
-            Count = allFilteredTransactions.Count,
-            TotalDebits = allFilteredTransactions.Where(t => t.Amount < 0).Sum(t => t.Amount),
-            TotalCredits = allFilteredTransactions.Where(t => t.Amount > 0).Sum(t => t.Amount),
-            NetAmount = allFilteredTransactions.Sum(t => t.Amount)
-        };
+        return stats ?? new TransactionStats();
     }
 
     public async Task<CategorizationStats> GetCategorizationStatsAsync()
@@ -64,24 +68,17 @@ public class TransactionStatisticsService : ITransactionStatisticsService
 
     public async Task<List<CardStats>> GetCardStatsForAccountAsync(int accountId)
     {
-        var cards = await _db.Cards
+        // Single query with projection to avoid N+1
+        return await _db.Cards
             .Where(c => c.AccountId == accountId)
             .OrderBy(c => c.Owner)
             .ThenBy(c => c.Last4)
-            .ToListAsync();
-
-        var cardStats = new List<CardStats>();
-        foreach (var card in cards)
-        {
-            var transactionCount = await _db.Transactions.CountAsync(t => t.CardId == card.Id);
-            cardStats.Add(new CardStats
+            .Select(c => new CardStats
             {
-                Card = card,
-                TransactionCount = transactionCount
-            });
-        }
-
-        return cardStats;
+                Card = c,
+                TransactionCount = c.Transactions.Count
+            })
+            .ToListAsync();
     }
 }