Add unending category for session-based games

For games like Dota 2 that have no completion state — shows hours
played on the card, reuses the games fields in the form (hours played
/ total hours), and gets its own cyan badge. No DB column change,
only choices metadata update.

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

backlogger/migrations/0006_item_category_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='category',
+            field=models.CharField(
+                choices=[
+                    ('games', 'Games'),
+                    ('books', 'Books'),
+                    ('films', 'Films'),
+                    ('unending', 'Unending'),
+                    ('other', 'Other'),
+                ],
+                max_length=10,
+            ),
+        ),
+    ]