fix: update MiniMax API endpoint from api.minimax.chat to api.minimax.io

The MiniMax API endpoint should be api.minimax.io, not api.minimax.chat.

See: https://platform.minimax.io/docs/api-reference/text-anthropic-api

Fixes #43

src/backend/app/services/ai_agent/llm_connector.py

@@ -8,7 +8,7 @@ class MiniMaxLLM(LLM):
         super().__init__(**kwargs)
         self.api_key = api_key
         self.model = model
-        self.base_url = "https://api.minimax.chat/v1"
+        self.base_url = "https://api.minimax.io/v1"
 
     def _call(self, messages: List[Dict[str, str]], **kwargs) -> str:
         headers = {