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.

src/Appointment/Entity/DateOverride.php

@@ -91,6 +91,9 @@ class DateOverride
             'clinic_uuid' => $this->clinic?->getUuid(),
             'context'     => $this->clinic === null ? 'personal' : 'clinic',
             'date'        => $this->date,
+            // نمایش تاریخ باید از این رشته خوانده شود؛ تبدیل timestamp در مرورگر
+            // با UTC انجام می‌شود و برای نیمه‌شبِ تهران یک روز عقب می‌افتد.
+            'date_string' => date('Y-m-d', $this->date),
             'active'      => $this->active,
             'reason'      => $this->reason,
             'custom_slots' => $this->setting ?? [],

src/Appointment/Entity/Holiday.php

@@ -90,6 +90,10 @@ class Holiday
             'scope'       => $this->clinic === null ? 'global' : 'clinic',
             'start_date'  => $this->startDate,
             'end_date'    => $this->endDate,
+            // نمایش تاریخ باید از این رشته‌ها خوانده شود؛ تبدیل timestamp در مرورگر
+            // با UTC انجام می‌شود و برای نیمه‌شبِ تهران یک روز عقب می‌افتد.
+            'start_date_string' => date('Y-m-d', $this->startDate),
+            'end_date_string'   => date('Y-m-d', $this->endDate),
             'active'      => $this->active,
             'reason'      => $this->reason,
             'created_at'  => $this->createdAt,