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>

notifications/tasks.py

@@ -236,3 +236,51 @@ def schedule_task_reminder(task_id):
                 task=task,
                 remind_at=remind_at
             )
+
+
+@shared_task
+def process_recurring_tasks():
+    """
+    Process completed recurring tasks and create next instances.
+    This runs periodically as a safety net to catch any tasks that weren't
+    automatically handled when marked as completed.
+    Runs every hour via Celery Beat.
+    """
+    from tasks.models import Task
+
+    # Find completed recurring tasks that don't have a next instance created yet
+    # We look for tasks completed in the last 24 hours to avoid reprocessing old tasks
+    yesterday = timezone.now() - timedelta(days=1)
+
+    completed_recurring_tasks = Task.objects.filter(
+        status='completed',
+        recurrence__in=['daily', 'weekly', 'biweekly', 'monthly', 'yearly', 'custom'],
+        completed_at__gte=yesterday
+    ).exclude(recurrence='none')
+
+    tasks_created = 0
+    for task in completed_recurring_tasks:
+        # Check if a next recurrence already exists
+        # Look for pending tasks with the same title, user, and recurrence pattern
+        next_due_date = task.calculate_next_due_date()
+        if next_due_date:
+            # Check if we already created this recurrence
+            existing = Task.objects.filter(
+                user=task.user,
+                title=task.title,
+                due_date=next_due_date,
+                recurrence=task.recurrence,
+                status='pending'
+            ).exists()
+
+            if not existing:
+                # Create the next recurrence
+                new_task = task.create_next_recurrence()
+                if new_task:
+                    tasks_created += 1
+                    logger.info(f"Created recurring task: {new_task.title} (due: {new_task.due_date})")
+
+    if tasks_created > 0:
+        logger.info(f"Created {tasks_created} recurring task instances")
+
+    return tasks_created