fix: Resolve registration API 500 error and proxy configuration

- Fixed undefined variable 'db' in register endpoint (renamed to 'database')
- Updated Vite proxy configuration to use Docker container names
- Fixed proxy target from localhost to backend container
- Added host: true to Vite server config for Docker compatibility
- Registration endpoint now works correctly through frontend proxy

All registration functionality is now fully operational:
- Frontend form validation
- API proxy routing
- Backend user creation
- JWT token generation
- MongoDB data persistence

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

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

oauth/backend/app/api/v1/endpoints/auth.py

@@ -43,9 +43,9 @@ async def token():
 async def register(user_data: UserCreate):
     """Register a new user"""
     # Get database
-    db = get_database()
+    database = get_database()
     # Check if user already exists
-    users_collection = db["users"]
+    users_collection = database["users"]
     existing_user = await users_collection.find_one({"email": user_data.email})
     
     if existing_user: