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/views.py

@@ -425,6 +425,9 @@ class TaskDetailView(View):
         task.title = request.POST.get('title', task.title)
         task.description = request.POST.get('description', '')
 
+        # Track old status to detect completion
+        old_status = task.status
+
         # Validate status against allowed choices
         status = request.POST.get('status', task.status)
         valid_statuses = [choice[0] for choice in Task.STATUS_CHOICES]
@@ -451,6 +454,10 @@ class TaskDetailView(View):
         else:
             task.recurrence = 'none'
 
+        # Create next recurrence if task is being marked as completed
+        if old_status != 'completed' and task.status == 'completed' and task.recurrence != 'none':
+            task.create_next_recurrence()
+
         task.save()
 
         # Handle tags
@@ -564,7 +571,13 @@ def task_toggle_status(request, task_id):
     if task.status == 'completed':
         task.status = 'pending'
     else:
+        # Mark as completed
         task.status = 'completed'
+
+        # Create next recurrence if this is a recurring task
+        if task.recurrence != 'none':
+            task.create_next_recurrence()
+
     task.save()
 
     next_url = request.POST.get('next', 'dashboard')