Fix SyntaxError and ImportError in model_router.py - async def + ModelRouter alias (908 lines)

backend/model_router.py

@@ -902,3 +902,7 @@ Provide comprehensive medical analysis with actionable clinical insights."""
     async def execute_task(self, task: Dict[str, Any]) -> Dict[str, Any]:
         """Legacy execute method for backward compatibility"""
         return await self.execute_research_optimized_task(task)
+
+
+# Backward compatibility alias for main.py import
+ModelRouter = EnhancedModelRouter