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/components/session/CreateStep.tsx

@@ -10,7 +10,7 @@ import SearchableSelect from '../ui/SearchableSelect';
 import PersianDateInput from '../ui/PersianDateInput';
 import { UserTick, FilesServiceAddCard, ClockP, TrashRed } from '../icons/FilesServiceIcons';
 import { useAuthStore } from '../../stores/authStore';
-import { digitsOnly } from '../../lib/utils';
+import { digitsOnly, todayIso } from '../../lib/utils';
 import {
   DEFAULT_SERVICE_CATEGORY, contractPercentFor, patientShareOf,
   type CoverageRule as Rule, type TenantContract,
@@ -29,7 +29,7 @@ const VISIT_SERVICE_CATEGORY = DEFAULT_SERVICE_CATEGORY;
 // مصرف‌کنندگان قبلی (و تست‌ها) نشکنند.
 export { patientShareOf };
 
-const todayISO = () => new Date().toISOString().slice(0, 10);
+const todayISO = todayIso;
 const nowHHMM = () => {
   const d = new Date();
   return `${String(d.getHours()).padStart(2, '0')}:${String(d.getMinutes()).padStart(2, '0')}`;