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/migrations/0005_item_status.py

@@ -0,0 +1,20 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('backlogger', '0004_item_hltb_fields'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='item',
+            name='status',
+            field=models.CharField(
+                choices=[('active', 'Active'), ('completed', 'Completed'), ('abandoned', 'Abandoned')],
+                default='active',
+                max_length=10,
+            ),
+        ),
+    ]