fix: 企微回调验证 — 返回纯文本而非 JSON

企微后台保存时要求响应为原始解密明文(Content-Type: text/plain)，
FastAPI 默认 JSON 序列化会加引号导致验证失败。
改用 PlainTextResponse 返回纯文本。

验证: 本地 + 生产域名均返回 text/plain, 解密内容正确

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

backend/app/api/wecom.py

@@ -4,6 +4,7 @@ import base64
 import uuid
 from typing import Optional
 from fastapi import APIRouter, Depends, HTTPException, Query, Request
+from fastapi.responses import PlainTextResponse
 from pydantic import BaseModel
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
@@ -73,7 +74,8 @@ async def wecom_callback_verify(
 
     try:
         plaintext = _decrypt_echostr(echostr)
-        return plaintext
+        # Must return raw plaintext (not JSON) — WeChat Work requires exact match
+        return PlainTextResponse(content=plaintext, status_code=200)
     except Exception as e:
         raise HTTPException(status_code=500, detail=f"Decrypt failed: {e}")