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/routers/query.py

@@ -7,6 +7,7 @@ from fastapi.responses import JSONResponse
 
 from models.schemas import APIResponse, BatchQueryRequest, QueryRequest
 from services import qa_service as svc
+from services.qa_service import KGEmptyError
 
 router = APIRouter(prefix="/query", tags=["QA"])
 
@@ -20,12 +21,12 @@ async def run_query(body: QueryRequest):
             partial(svc.run_query, body.question, [m.model_dump() for m in body.history]),
         )
         return APIResponse.ok(result)
+    except KGEmptyError as e:
+        return JSONResponse(
+            status_code=400,
+            content=APIResponse.err(3002, str(e)).model_dump(),
+        )
     except ValueError as e:
-        if "KG_EMPTY" in str(e):
-            return JSONResponse(
-                status_code=400,
-                content=APIResponse.err(3002, "Knowledge graph is empty. Index documents first.").model_dump(),
-            )
         return JSONResponse(
             status_code=500,
             content=APIResponse.err(4001, str(e)).model_dump(),