Fix loan interest calculation to use daily accrual instead of rate/12

The amortization schedule and payoff calculator were using rate/12 for
monthly interest, which assumes 30.42 days per period and overestimates
interest. Switch to daily accrual (rate/365 * actual days) to match how
lenders actually compute interest between payment dates.

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

src/Purrse.Api/Services/LoanService.cs

@@ -113,11 +113,12 @@ public class LoanService : ILoanService
 
         // Build new schedule with extra payments
         var balance = Math.Abs(account.Balance);
-        var monthlyRate = l.InterestRate / 100 / 12;
+        var dailyRate = l.InterestRate / 100m / 365m;
         var extraMonthly = request.ExtraMonthlyPayment ?? 0;
         var basePayment = l.MonthlyPayment - (l.EscrowAmount ?? 0);
         var now = DateTime.UtcNow;
-        var paymentDate = new DateTime(now.Year, now.Month, l.OriginationDate.Day, 0, 0, 0, DateTimeKind.Utc);
+        var startDate = new DateTime(now.Year, now.Month, l.OriginationDate.Day, 0, 0, 0, DateTimeKind.Utc);
+        var previousDate = startDate;
 
         var newEntries = new List<AmortizationEntryResponse>();
         int paymentNum = 0;
@@ -132,14 +133,16 @@ public class LoanService : ILoanService
         while (balance > 0 && paymentNum < 600)
         {
             paymentNum++;
-            paymentDate = paymentDate.AddMonths(1);
+            var paymentDate = startDate.AddMonths(paymentNum);
+            var days = (paymentDate - previousDate).Days;
 
-            var interest = balance * monthlyRate;
+            var interest = balance * dailyRate * days;
             var principal = Math.Min(basePayment + extraMonthly - interest, balance);
             if (principal < 0) principal = 0;
 
             balance -= principal;
             if (balance < 0.01m) balance = 0;
+            previousDate = paymentDate;
 
             newEntries.Add(new AmortizationEntryResponse(
                 paymentNum, paymentDate, principal + interest + (l.EscrowAmount ?? 0),
@@ -180,18 +183,20 @@ public class LoanService : ILoanService
     {
         var entries = new List<AmortizationEntry>();
         var balance = loan.OriginalBalance;
-        var monthlyRate = loan.InterestRate / 100 / 12;
+        var dailyRate = loan.InterestRate / 100m / 365m;
         var payment = loan.MonthlyPayment - (loan.EscrowAmount ?? 0);
         var extra = loan.ExtraPayment ?? 0;
-        var paymentDate = loan.OriginationDate;
+        var previousDate = loan.OriginationDate;
 
         for (int i = 1; i <= loan.TermMonths && balance > 0; i++)
         {
-            paymentDate = paymentDate.AddMonths(1);
+            var paymentDate = loan.OriginationDate.AddMonths(i);
-            var interest = balance * monthlyRate;
+            var days = (paymentDate - previousDate).Days;
+            var interest = balance * dailyRate * days;
             var principal = Math.Min(payment + extra - interest, balance);
             balance -= principal;
             if (balance < 0.01m) balance = 0;
+            previousDate = paymentDate;
 
             entries.Add(new AmortizationEntry
             {