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

@@ -0,0 +1,25 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('backlogger', '0005_item_status'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='item',
+            name='status',
+            field=models.CharField(
+                choices=[
+                    ('active', 'Active'),
+                    ('completed', 'Completed'),
+                    ('abandoned', 'Abandoned'),
+                    ('unending', 'Unending'),
+                ],
+                default='active',
+                max_length=10,
+            ),
+        ),
+    ]