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>

users/throttles.py

@@ -0,0 +1,32 @@
+"""
+Custom throttle classes for user-related endpoints.
+"""
+
+from rest_framework.throttling import AnonRateThrottle, UserRateThrottle
+
+
+class LoginRateThrottle(AnonRateThrottle):
+    """
+    Rate limit for login attempts.
+
+    Stricter than general anonymous rate to prevent brute force attacks.
+    """
+    scope = 'login'
+
+
+class RegisterRateThrottle(AnonRateThrottle):
+    """
+    Rate limit for registration attempts.
+
+    Prevents automated account creation and abuse.
+    """
+    scope = 'register'
+
+
+class SyncRateThrottle(UserRateThrottle):
+    """
+    Rate limit for sync endpoint.
+
+    Prevents excessive sync requests that could overload the server.
+    """
+    scope = 'sync'