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>

config/settings/base.py

@@ -123,6 +123,17 @@ REST_FRAMEWORK = {
     ],
     'DEFAULT_PAGINATION_CLASS': 'rest_framework.pagination.PageNumberPagination',
     'PAGE_SIZE': 50,
+    'DEFAULT_THROTTLE_CLASSES': [
+        'rest_framework.throttling.AnonRateThrottle',
+        'rest_framework.throttling.UserRateThrottle',
+    ],
+    'DEFAULT_THROTTLE_RATES': {
+        'anon': '100/hour',  # Anonymous users: 100 requests per hour
+        'user': '1000/hour',  # Authenticated users: 1000 requests per hour
+        'login': '10/hour',  # Login attempts: 10 per hour
+        'register': '5/hour',  # Registration attempts: 5 per hour
+        'sync': '100/hour',  # Sync endpoint: 100 per hour
+    },
 }
 
 # JWT Settings