Add completed/abandoned shelves with status transitions

Items can be moved to Completed or Abandoned via card buttons.
Only active items appear in the default/category tabs; completed and
abandoned items are visible only in their respective shelf tabs.
Restore button moves items back to active.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backlogger/models.py

@@ -14,9 +14,19 @@ class Item(models.Model):
         (OTHER, 'Other'),
     ]
 
+    ACTIVE = 'active'
+    COMPLETED = 'completed'
+    ABANDONED = 'abandoned'
+    STATUS_CHOICES = [
+        (ACTIVE, 'Active'),
+        (COMPLETED, 'Completed'),
+        (ABANDONED, 'Abandoned'),
+    ]
+
     user = models.ForeignKey(User, on_delete=models.CASCADE, related_name='items', null=True)
 
     category = models.CharField(max_length=10, choices=CATEGORY_CHOICES)
+    status = models.CharField(max_length=10, choices=STATUS_CHOICES, default=ACTIVE)
     name = models.CharField(max_length=200)
     progress_percent = models.FloatField(default=0.0)
     favorite = models.BooleanField(default=False)