fix: fetch_fund_flow_daykline 增加 rc 校验和 data 空值保护

东方财富 daykline/get 接口返回 rc=100(data=null) 时无保护处理：
- 增加 result 类型检查、rc 状态校验、data 空值判断
- 避免 None.get('klines') 抛 AttributeError

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

backend/services/eastmoney.py

@@ -1074,7 +1074,20 @@ async def fetch_fund_flow_daykline(code: str, days: int) -> Optional[list[dict]]
                         continue
                     return None
                 result = resp.json()
-                klines = result.get("data", {}).get("klines", [])
+                if not isinstance(result, dict):
+                    if attempt == 0:
+                        continue
+                    return None
+                if result.get("rc", -1) != 0:
+                    if attempt == 0:
+                        continue
+                    return None
+                data = result.get("data")
+                if not data or not isinstance(data, dict):
+                    if attempt == 0:
+                        continue
+                    return None
+                klines = data.get("klines", [])
                 if not klines:
                     if attempt == 0:
                         continue