perf: 系统级异步邮件 — 消除SMTP阻塞导致的10-15秒UI冻结

## email_service.py
- 新增 send_email_async(): threading.Thread(daemon=True) 包装 send_email()
  无需 Flask app_context (send_email仅用os.getenv+smtplib)
- 6处 send_email→send_email_async 替换

## inventory_task.py
- 库存预警 send_email→send_email_async (2处)

inventory-backend/app/services/inventory_task.py

@@ -129,7 +129,7 @@ class InventoryWarningService:
                 "timestamp": "..."
             }
         """
-        from app.utils.email_service import send_email
+        from app.utils.email_service import send_email_async
 
         beijing_tz = timezone(timedelta(hours=8))
         now = datetime.now(beijing_tz)
@@ -248,7 +248,7 @@ class InventoryWarningService:
                 "详情请登录仓库管理系统查看。\n\n"
                 "此邮件由系统自动发送，请勿回复。"
             )
-            send_email(email, subject, content)
+            send_email_async(email, subject, content)
             sent_red = True
 
         # ★ 按邮箱聚合，批量发送黄色预警邮件 ★
@@ -262,7 +262,7 @@ class InventoryWarningService:
                 "详情请登录仓库管理系统查看。\n\n"
                 "此邮件由系统自动发送，请勿回复。"
             )
-            send_email(email, subject, content)
+            send_email_async(email, subject, content)
             sent_yellow = True
 
         # ★ 批量更新 last_notified_at ★