Fix unending: shelf status, not category

Unending belongs in STATUS_CHOICES alongside completed/abandoned.
Adds an Unending shelf tab and an ∞ button on active game cards.
Reverts the incorrect category addition from the previous commit.

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

backlogger/migrations/0006_item_status_unending.py

@@ -10,15 +10,15 @@ class Migration(migrations.Migration):
     operations = [
         migrations.AlterField(
             model_name='item',
-            name='category',
+            name='status',
             field=models.CharField(
                 choices=[
-                    ('games', 'Games'),
-                    ('books', 'Books'),
-                    ('films', 'Films'),
+                    ('active', 'Active'),
+                    ('completed', 'Completed'),
+                    ('abandoned', 'Abandoned'),
                     ('unending', 'Unending'),
-                    ('other', 'Other'),
                 ],
+                default='active',
                 max_length=10,
             ),
         ),