feat: add initial Django domain models

backend/checkins/models.py

@@ -1 +1,46 @@
-# Domain models will be added when check-in behavior is implemented.
+from django.conf import settings
+from django.core.exceptions import ValidationError
+from django.db import models
+from django.utils import timezone
+
+from bookings.models import Reservation
+
+
+class CheckIn(models.Model):
+    class Source(models.TextChoices):
+        QR_SCAN = "qr_scan", "QR scan"
+        MANUAL = "manual", "Manual"
+
+    reservation = models.OneToOneField(
+        Reservation,
+        on_delete=models.PROTECT,
+        related_name="check_in",
+    )
+    checked_in_at = models.DateTimeField(default=timezone.now)
+    checked_in_by = models.ForeignKey(
+        settings.AUTH_USER_MODEL,
+        on_delete=models.PROTECT,
+        related_name="checkins",
+    )
+    source = models.CharField(max_length=20, choices=Source.choices, default=Source.QR_SCAN)
+    created_at = models.DateTimeField(auto_now_add=True)
+    updated_at = models.DateTimeField(auto_now=True)
+
+    class Meta:
+        ordering = ["-checked_in_at"]
+        indexes = [
+            models.Index(fields=["checked_in_at"]),
+            models.Index(fields=["checked_in_by"]),
+        ]
+
+    def __str__(self):
+        return f"Check-in for reservation {self.reservation_id}"
+
+    def clean(self):
+        super().clean()
+        if self.reservation_id and not self.reservation.is_confirmed:
+            raise ValidationError({"reservation": "Only confirmed reservations can be checked in."})
+
+    def save(self, *args, **kwargs):
+        self.full_clean()
+        return super().save(*args, **kwargs)