feat(booking): add validation to prevent past bookings and implement slot validation logic

apps/backend/src/lib/slot-validator.ts

@@ -0,0 +1,34 @@
+function toMinutes(value: string): number {
+  const [hours, minutes] = value.split(':').map((part) => Number(part));
+  return hours * 60 + minutes;
+}
+
+export function isSlotInPast(
+  bookingDate: Date,
+  startTime: string,
+  slotDurationMinutes: number,
+  now?: Date
+): boolean {
+  const currentTime = now ?? new Date();
+
+  const todayStart = new Date(
+    currentTime.getFullYear(),
+    currentTime.getMonth(),
+    currentTime.getDate()
+  );
+
+  const bookingLocalStart = new Date(
+    bookingDate.getUTCFullYear(),
+    bookingDate.getUTCMonth(),
+    bookingDate.getUTCDate()
+  );
+
+  if (bookingLocalStart < todayStart) return true;
+  if (bookingLocalStart > todayStart) return false;
+
+  const currentMinutes = currentTime.getHours() * 60 + currentTime.getMinutes();
+  const slotStartMinutes = toMinutes(startTime);
+  const elapsed = currentMinutes - slotStartMinutes;
+
+  return elapsed > slotDurationMinutes / 2;
+}