Replace pytz with zoneinfo for timezone handling

Fixes ModuleNotFoundError by using Python's built-in zoneinfo module
instead of the external pytz dependency. zoneinfo is the standard
library solution for timezone handling in Python 3.9+.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

tasks/models.py

@@ -153,14 +153,14 @@ class Task(models.Model):
         """Check if task is overdue."""
         if self.due_date and self.status not in ('completed', 'cancelled'):
             from django.utils import timezone
-            import pytz
+            from zoneinfo import ZoneInfo
 
             # Get current date in user's timezone
             try:
-                user_tz = pytz.timezone(self.user.timezone)
+                user_tz = ZoneInfo(self.user.timezone)
                 user_now = timezone.now().astimezone(user_tz)
                 user_today = user_now.date()
-            except (pytz.UnknownTimeZoneError, AttributeError):
+            except (Exception,):
                 # Fall back to UTC if user timezone is invalid or missing
                 user_today = timezone.now().date()