bugfix: appointment hours now use the client timezone on UTC servers.

Logical API errors throw stable codes so users see translated messages instead of a generic bad request.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/src/common/zoned-civil-time.spec.ts

@@ -0,0 +1,29 @@
+import { appointmentWithinWorkingHours } from './working-hours';
+import { isValidIanaTimeZone, zonedWeekdayAndMinutes } from './zoned-civil-time';
+
+describe('zoned civil time', () => {
+  it('accepts IANA zones and rejects garbage', () => {
+    expect(isValidIanaTimeZone('Asia/Tehran')).toBe(true);
+    expect(isValidIanaTimeZone('Europe/Amsterdam')).toBe(true);
+    expect(isValidIanaTimeZone('Not/AZone')).toBe(false);
+  });
+
+  it('maps a UTC instant to Tehran wall-clock (no DST)', () => {
+    // 2026-08-20 09:45 Asia/Tehran = 06:15 UTC (the production 400 case)
+    const instant = new Date('2026-08-20T06:15:00.000Z');
+    const zoned = zonedWeekdayAndMinutes(instant, 'Asia/Tehran');
+    expect(zoned.jsWeekday).toBe(4); // Thursday
+    expect(zoned.minuteOfDay).toBe(9 * 60 + 45);
+  });
+
+  it('treats morning Tehran slots as inside 08:00–17:00 hours', () => {
+    const start = new Date('2026-08-20T06:15:00.000Z');
+    const end = new Date('2026-08-20T06:45:00.000Z');
+    expect(
+      appointmentWithinWorkingHours(start, end, [{ startMinute: 8 * 60, endMinute: 17 * 60 }], 'Asia/Tehran'),
+    ).toBe(true);
+    expect(
+      appointmentWithinWorkingHours(start, end, [{ startMinute: 8 * 60, endMinute: 17 * 60 }], 'UTC'),
+    ).toBe(false);
+  });
+});