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/models.py

@@ -6,11 +6,13 @@ class Item(models.Model):
     GAMES = 'games'
     BOOKS = 'books'
     FILMS = 'films'
+    UNENDING = 'unending'
     OTHER = 'other'
     CATEGORY_CHOICES = [
         (GAMES, 'Games'),
         (BOOKS, 'Books'),
         (FILMS, 'Films'),
+        (UNENDING, 'Unending'),
         (OTHER, 'Other'),
     ]