Fix SQLite AVG() type conversion in photo size stats

- Convert avg_size_bytes to float to handle SQLite returning string values
- Fixes TypeError: '>' not supported between instances of 'str' and 'int'
- Ensures photo size statistics work correctly across different database backends
- All 5 stats tests pass successfully

The issue occurred because SQLite's AVG() function can return string representations
of numbers in some cases, causing type comparison errors in tests.

ivatar/views.py

@@ -698,6 +698,8 @@ class StatsView(TemplateView, JsonResponse):
 
             if result and result[0] > 0:
                 photo_count, avg_size_bytes = result
+                # Convert to float in case SQLite returns string
+                avg_size_bytes = float(avg_size_bytes) if avg_size_bytes else 0
                 avg_size_kb = round(avg_size_bytes / 1024, 2) if avg_size_bytes else 0
                 avg_size_mb = (
                     round(avg_size_bytes / (1024 * 1024), 2) if avg_size_bytes else 0