Fix concurrency exception when editing loan details

RemoveRange + collection replacement caused EF Core to attempt a
double-delete on amortization entries. Use Clear() on the tracked
collection instead, letting EF handle orphan deletion in a single pass.

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

src/Purrse.Api/Services/LoanService.cs

@@ -70,7 +70,7 @@ public class LoanService : ILoanService
 
         var l = account.LoanDetail ?? throw new KeyNotFoundException("No loan detail found");
 
-        _db.AmortizationEntries.RemoveRange(l.AmortizationEntries);
+        l.AmortizationEntries.Clear();
 
         l.OriginalBalance = request.OriginalBalance;
         l.InterestRate = request.InterestRate;
@@ -82,7 +82,8 @@ public class LoanService : ILoanService
         l.ExtraPayment = request.ExtraPayment;
 
         var entries = GenerateAmortizationSchedule(l);
-        l.AmortizationEntries = entries;
+        foreach (var entry in entries)
+            l.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();