fix(auth): mp-qr失败时返回详细错误信息，便于排查小程序码生成失败

- /auth/mp-qr/start 和 /auth/mp-qr/bind-start 在 get_wxacode 失败时
  返回 error 字段，包含具体失败原因（缺少 WECHAT_APPID/SECRET 或微信API调用失败）
- 之前只返回 mp_enabled:false + qr_url，前端无法区分是配置问题还是API问题

app/api/routers/auth.py

@@ -743,10 +743,11 @@ async def mp_qr_start(request: Request, db: Database = Depends(get_db)):
         png = await wechat.get_wxacode(scene, page="pages/scan-login/index")
         b64 = base64.b64encode(png).decode("ascii")
         return {"scene": scene, "qr_image": f"data:image/png;base64,{b64}", "mp_enabled": True, "expires_in": wx_qr_store.TTL_SECONDS}
-    except wechat.WechatError:
+    except wechat.WechatError as exc:
         # 无凭据/失败兜底：返回可扫描的 URL，前端按 qr_url 渲染普通二维码
+        # 同时返回错误详情，便于排查（服务器缺少 WECHAT_APPID/SECRET 或微信API调用失败）
         qr_url = f"https://opc.pinesound.cn/mp/login?scene={scene}"
-        return {"scene": scene, "qr_url": qr_url, "mp_enabled": False, "expires_in": wx_qr_store.TTL_SECONDS}
+        return {"scene": scene, "qr_url": qr_url, "mp_enabled": False, "error": str(exc), "expires_in": wx_qr_store.TTL_SECONDS}
 
 
 @router.get("/mp-qr/poll", summary="轮询小程序扫码登录状态")
@@ -773,9 +774,9 @@ async def mp_qr_bind_start(request: Request, db: Database = Depends(get_db), use
         png = await wechat.get_wxacode(scene, page="pages/scan-login/index")
         b64 = base64.b64encode(png).decode("ascii")
         return {"scene": scene, "qr_image": f"data:image/png;base64,{b64}", "mp_enabled": True, "expires_in": wx_qr_store.TTL_SECONDS}
-    except wechat.WechatError:
+    except wechat.WechatError as exc:
         qr_url = f"https://opc.pinesound.cn/mp/bind?scene={scene}"
-        return {"scene": scene, "qr_url": qr_url, "mp_enabled": False, "expires_in": wx_qr_store.TTL_SECONDS}
+        return {"scene": scene, "qr_url": qr_url, "mp_enabled": False, "error": str(exc), "expires_in": wx_qr_store.TTL_SECONDS}
 
 
 @router.post("/mp-qr/confirm", summary="小程序内确认扫码登录/绑定")