Created and activated the booking integrity ExecPlan, then implemented staff availability, overlap prevention, and duration validation with backend tests.

Added a staff availability model and migration, a booking validation service, and serializer enforcement.

backend/apps/bookings/services.py

@@ -0,0 +1,45 @@
+from datetime import timedelta
+
+from django.utils.translation import gettext_lazy as _
+from rest_framework import serializers
+
+from apps.bookings.models import Booking, BookingStatus
+from apps.salons.models import StaffAvailability, StaffProfile
+
+
+def validate_booking_request(service, staff, start_time, end_time):
+    if staff is None:
+        raise serializers.ValidationError({"staff": _("Staff is required for booking")})
+
+    if isinstance(staff, StaffProfile) and staff.salon_id != service.salon_id:
+        raise serializers.ValidationError({"staff": _("Selected staff does not belong to this salon")})
+
+    if start_time >= end_time:
+        raise serializers.ValidationError({"end_time": _("End time must be after start time")})
+
+    expected_end = start_time + timedelta(minutes=service.duration_minutes)
+    if expected_end != end_time:
+        raise serializers.ValidationError({"end_time": _("End time must match service duration")})
+
+    availability_qs = StaffAvailability.objects.filter(
+        staff=staff,
+        day_of_week=start_time.weekday(),
+        is_active=True,
+    )
+
+    if availability_qs.exists():
+        within_window = availability_qs.filter(
+            start_time__lte=start_time.time(),
+            end_time__gte=end_time.time(),
+        ).exists()
+        if not within_window:
+            raise serializers.ValidationError({"start_time": _("Booking is outside staff availability")})
+
+    overlap_exists = Booking.objects.filter(
+        staff=staff,
+        status__in=[BookingStatus.PENDING, BookingStatus.CONFIRMED],
+        start_time__lt=end_time,
+        end_time__gt=start_time,
+    ).exists()
+    if overlap_exists:
+        raise serializers.ValidationError({"start_time": _("Booking overlaps an existing appointment")})