fix: KGEmptyError 自定义异常 + CORS 环境变量配置

- qa_service 定义 KGEmptyError 异常，run_query 抛出；routers/query 精确捕获 → HTTP 400 + code 3002，移除字符串匹配
- main.py CORS 改为读 CORS_ORIGINS 环境变量：未设置时 wildcard + credentials=False（合规默认），设置后显式列表 + credentials=True
- .env.example 新增 CORS_ORIGINS 默认值（后端 + 前端 4 个域名）

backend/services/qa_service.py

@@ -8,6 +8,13 @@ from datetime import datetime, timezone
 from storage import file_store as fs
 
 
+class KGEmptyError(Exception):
+    """Raised when a QA request is made but the knowledge graph has no nodes."""
+
+    def __init__(self, msg: str = "Knowledge graph is empty. Index documents first."):
+        super().__init__(msg)
+
+
 def run_query(question: str, history: list[dict]) -> dict:
     from pipeline.qa_agent import run_qa
 
@@ -15,7 +22,7 @@ def run_query(question: str, history: list[dict]) -> dict:
     edges = fs.load_kg_edges()
 
     if not nodes:
-        raise ValueError("KG_EMPTY")
+        raise KGEmptyError()
 
     start = time.time()
     result = run_qa(question, history, nodes, edges)