refactor: normalize date handling to Tehran timezone

- Updated date handling in BlogSeoFields and ScheduleSection to use Tehran timezone utilities for consistency.
- Introduced `toTehranClockTime`, `tehranWallClockToUnix`, and `todayIso` functions for accurate date representation.
- Modified various components to utilize these new utilities, ensuring that date strings are correctly formatted and timestamps are accurately converted.
- Enhanced API documentation to clarify the handling of date fields, emphasizing the importance of server-local midnight.
- Added tests to verify that date overrides and holidays maintain the correct day without shifting due to timezone discrepancies.

assets/admin/pages/MyPaymentsPage.tsx

@@ -7,7 +7,7 @@ import SearchableSelect from '../components/ui/SearchableSelect';
 import StatCard from '../components/ui/StatCard';
 import StatusBadge from '../components/ui/StatusBadge';
 import DataTable, { type Column } from '../components/ui/DataTable';
-import { formatRial, formatDate, formatNumber, toDate } from '../lib/utils';
+import { formatRial, formatDate, formatNumber, toDate, toGregorianDate, todayIso } from '../lib/utils';
 import {
   usePayments,
   usePaymentsSummary,
@@ -25,9 +25,8 @@ const STATUS_OPTIONS = [
 const isoNDaysAgo = (days: number): string => {
   const d = new Date();
   d.setDate(d.getDate() - days);
-  return d.toISOString().slice(0, 10);
+  return toGregorianDate(d);
 };
-const todayIso = (): string => new Date().toISOString().slice(0, 10);
 
 /** HH:MM (Persian digits) from a unix timestamp. */
 function formatTime(unix: number): string {