Add BackgroundImage model and enhance card templates with dynamic backgrounds

cards/models.py

@@ -33,3 +33,29 @@ class Dinosaur(models.Model):
 
     def __str__(self):
         return self.name
+
+
+class BackgroundImage(models.Model):
+    """Background images used behind card renders.
+
+    Admins can upload multiple backgrounds and mark one as active. Templates
+    will use BackgroundImage.get_active() to obtain the active background.
+    """
+    title = models.CharField(max_length=200, blank=True)
+    image = models.ImageField(upload_to='backgrounds/')
+    is_active = models.BooleanField(default=False, help_text='Mark as the active background')
+    uploaded_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        ordering = ['-is_active', '-uploaded_at']
+
+    def __str__(self):
+        return self.title or f"Background #{self.pk}"
+
+    @classmethod
+    def get_active(cls):
+        """Return the active BackgroundImage if set, otherwise the most recent one or None."""
+        active = cls.objects.filter(is_active=True).order_by('-uploaded_at').first()
+        if active:
+            return active
+        return cls.objects.order_by('-uploaded_at').first()