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/migrations/0004_item_hltb_fields.py

@@ -0,0 +1,26 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('backlogger', '0003_assign_existing_items_to_first_user'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='item',
+            name='hltb_main',
+            field=models.FloatField(blank=True, null=True),
+        ),
+        migrations.AddField(
+            model_name='item',
+            name='hltb_extra',
+            field=models.FloatField(blank=True, null=True),
+        ),
+        migrations.AddField(
+            model_name='item',
+            name='hltb_complete',
+            field=models.FloatField(blank=True, null=True),
+        ),
+    ]