Normalize all DateTime values to UTC across the codebase

Npgsql requires DateTimeKind.Utc for timestamp with time zone
columns. This fixes errors in transaction create/edit, budgets,
reports, loans, scheduled transactions, and file imports.

Changes:
- TransactionService: SpecifyKind on request.Date for create,
  update, and transfer
- BudgetService: UTC kind on all new DateTime(year, month, 1)
- ReportService: UTC kind on endOfMonth; rewrite spending-by-
  category GroupBy to avoid untranslatable navigation properties
- LoanService: UTC kind on payoff scenario date construction
- ScheduledTransactionService: UTC kind on SemiMonthly date
- ImportService: SpecifyKind on parsed.Date when persisting
- OFX/CSV/QIF parsers: AssumeUniversal + AdjustToUniversal on
  all date parsing

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

src/Purrse.Api/Services/LoanService.cs

@@ -115,8 +115,8 @@ public class LoanService : ILoanService
         var monthlyRate = l.InterestRate / 100 / 12;
         var extraMonthly = request.ExtraMonthlyPayment ?? 0;
         var basePayment = l.MonthlyPayment - (l.EscrowAmount ?? 0);
-        var paymentDate = DateTime.UtcNow.Date;
-        paymentDate = new DateTime(paymentDate.Year, paymentDate.Month, l.OriginationDate.Day);
+        var now = DateTime.UtcNow;
+        var paymentDate = new DateTime(now.Year, now.Month, l.OriginationDate.Day, 0, 0, 0, DateTimeKind.Utc);
 
         var newEntries = new List<AmortizationEntryResponse>();
         int paymentNum = 0;