Implement recurring tasks functionality

Adds automatic creation of next task instance when recurring tasks are completed.

- Add calculate_next_due_date() and create_next_recurrence() methods to Task model
- Update task completion handlers in views and sync API to create next recurrence
- Add hourly Celery task to process any missed recurring tasks
- Support daily, weekly, biweekly, monthly, yearly recurrence patterns
- Respect recurrence_end_date limits

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

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

tasks/models.py

@@ -174,6 +174,79 @@ class Task(models.Model):
         secs = seconds % 60
         return f"{hours}:{minutes:02d}:{secs:02d}"
 
+    def calculate_next_due_date(self, from_date=None):
+        """
+        Calculate the next due date based on recurrence pattern.
+        Returns None if task doesn't recur or has reached end date.
+        """
+        if self.recurrence == 'none':
+            return None
+
+        from datetime import timedelta
+        from dateutil.relativedelta import relativedelta
+
+        base_date = from_date or self.due_date
+        if not base_date:
+            from django.utils import timezone
+            base_date = timezone.now().date()
+
+        # Calculate next date based on recurrence type
+        if self.recurrence == 'daily':
+            next_date = base_date + timedelta(days=1)
+        elif self.recurrence == 'weekly':
+            next_date = base_date + timedelta(weeks=1)
+        elif self.recurrence == 'biweekly':
+            next_date = base_date + timedelta(weeks=2)
+        elif self.recurrence == 'monthly':
+            next_date = base_date + relativedelta(months=1)
+        elif self.recurrence == 'yearly':
+            next_date = base_date + relativedelta(years=1)
+        elif self.recurrence == 'custom' and self.recurrence_rule:
+            # TODO: Implement RRULE parsing for custom recurrence
+            # For now, default to weekly
+            next_date = base_date + timedelta(weeks=1)
+        else:
+            return None
+
+        # Check if we've passed the end date
+        if self.recurrence_end_date and next_date > self.recurrence_end_date:
+            return None
+
+        return next_date
+
+    def create_next_recurrence(self):
+        """
+        Create the next instance of this recurring task.
+        Returns the new task or None if no recurrence should be created.
+        """
+        if self.recurrence == 'none':
+            return None
+
+        next_due_date = self.calculate_next_due_date()
+        if not next_due_date:
+            return None
+
+        # Create new task with same properties
+        new_task = Task.objects.create(
+            user=self.user,
+            parent=self.parent,
+            title=self.title,
+            description=self.description,
+            status='pending',
+            priority=self.priority,
+            due_date=next_due_date,
+            due_time=self.due_time,
+            recurrence=self.recurrence,
+            recurrence_rule=self.recurrence_rule,
+            recurrence_end_date=self.recurrence_end_date,
+            sort_order=self.sort_order,
+        )
+
+        # Copy tags
+        new_task.tags.set(self.tags.all())
+
+        return new_task
+
 
 class TimeEntry(models.Model):
     """