Fix Critical Security Issue: Implement rate limiting on API endpoints

Added comprehensive rate limiting to prevent abuse and DoS attacks:

Configuration (config/settings/base.py):
- Anon rate: 100 requests/hour for anonymous users
- User rate: 1000 requests/hour for authenticated users
- Login rate: 10 attempts/hour (prevents brute force)
- Register rate: 5 attempts/hour (prevents account spam)
- Sync rate: 100 requests/hour (prevents sync flooding)

Custom throttle classes (users/throttles.py):
- LoginRateThrottle: Strict limit for login endpoint
- RegisterRateThrottle: Strict limit for registration endpoint
- SyncRateThrottle: Moderate limit for sync endpoint

Applied throttling to sensitive endpoints:
- users.views.RegisterAPIView: 5/hour
- users.views.TokenObtainPairView: 10/hour
- sync.views.sync: 100/hour

All other API endpoints use default throttles (anon: 100/hour, user: 1000/hour).

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

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

sync/views.py

@@ -10,7 +10,7 @@ from datetime import datetime
 from django.utils import timezone
 from django.utils.dateparse import parse_datetime
 from rest_framework import status
-from rest_framework.decorators import api_view, permission_classes
+from rest_framework.decorators import api_view, permission_classes, throttle_classes
 from rest_framework.permissions import IsAuthenticated
 from rest_framework.response import Response
 
@@ -25,12 +25,14 @@ from tasks.serializers import (
 )
 from .models import SyncLog, SyncConflict
 from .serializers import SyncConflictSerializer
+from users.throttles import SyncRateThrottle
 
 logger = logging.getLogger(__name__)
 
 
 @api_view(['POST'])
 @permission_classes([IsAuthenticated])
+@throttle_classes([SyncRateThrottle])
 def sync(request):
     """
     Main sync endpoint.