Add HowLongToBeat estimates to game cards

- Fetch HLTB main/extra/completionist hours when a game item is saved
- Re-fetch only when name or category changes on edit
- Steam imports also fetch HLTB for each selected game
- Cards show compact HLTB row: "HLTB: 40h · +extra 60h · 100% 100h"
- Edit form shows HLTB breakdown as a hint next to Total hours field

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

backlogger/models.py

@@ -35,6 +35,11 @@ class Item(models.Model):
     watched = models.BooleanField(null=True, blank=True)
     duration_minutes = models.IntegerField(null=True, blank=True)
 
+    # HowLongToBeat estimates (games only)
+    hltb_main = models.FloatField(null=True, blank=True)
+    hltb_extra = models.FloatField(null=True, blank=True)
+    hltb_complete = models.FloatField(null=True, blank=True)
+
     class Meta:
         ordering = ['-favorite', 'name']