Fix loan edit concurrency error with ExecuteDeleteAsync

The change tracker-based approaches (RemoveRange, Clear) both caused
DbUpdateConcurrencyException when deleting amortization entries.
Use ExecuteDeleteAsync to delete directly in the database, bypassing
the change tracker entirely.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Purrse.Api/Services/LoanService.cs

@@ -64,13 +64,14 @@ public class LoanService : ILoanService
 
     public async Task<LoanDetailResponse> UpdateLoanDetailAsync(Guid userId, Guid accountId, CreateLoanDetailRequest request)
     {
-        var account = await _db.Accounts.Include(a => a.LoanDetail).ThenInclude(l => l!.AmortizationEntries)
+        var account = await _db.Accounts.Include(a => a.LoanDetail)
             .FirstOrDefaultAsync(a => a.Id == accountId && a.UserId == userId)
             ?? throw new KeyNotFoundException("Account not found");
 
         var l = account.LoanDetail ?? throw new KeyNotFoundException("No loan detail found");
 
-        l.AmortizationEntries.Clear();
+        // Delete old entries directly in DB, bypassing change tracker
+        await _db.AmortizationEntries.Where(e => e.LoanDetailId == l.Id).ExecuteDeleteAsync();
 
         l.OriginalBalance = request.OriginalBalance;
         l.InterestRate = request.InterestRate;
@@ -83,7 +84,10 @@ public class LoanService : ILoanService
 
         var entries = GenerateAmortizationSchedule(l);
         foreach (var entry in entries)
-            l.AmortizationEntries.Add(entry);
+        {
+            entry.LoanDetailId = l.Id;
+            _db.AmortizationEntries.Add(entry);
+        }
         await _db.SaveChangesAsync();
 
         var schedule = entries.Select(e => new AmortizationEntryResponse(e.PaymentNumber, e.PaymentDate, e.PaymentAmount, e.PrincipalAmount, e.InterestAmount, e.EscrowAmount, e.RemainingBalance)).ToList();