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 ★

inventory-backend/app/utils/email_service.py

@@ -118,6 +118,22 @@ def send_email(to_email: Union[str, List[str]], subject: str, content: str):
         logger.error(f"[Email] 发送邮件时发生未知异常: {e}")
 
 
+def send_email_async(to_email: Union[str, List[str]], subject: str, content: str):
+    """
+    异步发送邮件（守护线程，不阻塞主请求线程）。
+
+    用法：直接替代 send_email() 调用，接口完全相同。
+    线程内不依赖 Flask context（仅用 os.getenv + smtplib），无需 app.app_context()。
+    """
+    import threading
+    t = threading.Thread(
+        target=send_email,
+        args=(to_email, subject, content),
+        daemon=True
+    )
+    t.start()
+
+
 def send_outbound_new_request_notify(to_emails: List[str], request_no: str,
                              applicant_name: str = '', remark: str = '',
                              items: list = None, is_applicant_notify: bool = False):
@@ -180,7 +196,7 @@ https://172.16.0.198/outbound/approval
 
 此邮件由系统自动发送，请勿回复。
 """
-    send_email(to_emails, subject, content)
+    send_email_async(to_emails, subject, content)
 
 
 def send_borrow_new_request_notify(to_emails: List[str], request_no: str,
@@ -245,7 +261,7 @@ https://172.16.0.198/operation/borrow_approval
 
 此邮件由系统自动发送，请勿回复。
 """
-    send_email(to_emails, subject, content)
+    send_email_async(to_emails, subject, content)
 
 
 def send_outbound_approval_result_notify(to_emails: List[str], request_no: str,
@@ -276,7 +292,7 @@ def send_outbound_approval_result_notify(to_emails: List[str], request_no: str,
 
 此邮件由系统自动发送，请勿回复。
 """
-    send_email(to_emails, subject, content)
+    send_email_async(to_emails, subject, content)
 
 
 def send_borrow_approval_result_notify(to_emails: List[str], request_no: str,
@@ -307,7 +323,7 @@ def send_borrow_approval_result_notify(to_emails: List[str], request_no: str,
 
 此邮件由系统自动发送，请勿回复。
 """
-    send_email(to_emails, subject, content)
+    send_email_async(to_emails, subject, content)
 
 
 def send_outbound_dispatch_notify(to_emails: List[str], request_no: str,
@@ -343,7 +359,7 @@ def send_outbound_dispatch_notify(to_emails: List[str], request_no: str,
 
 此邮件由系统自动发送，请勿回复。
 """
-    send_email(to_emails, subject, content)
+    send_email_async(to_emails, subject, content)
 
 
 def send_borrow_dispatch_notify(to_emails: List[str], request_no: str,
@@ -379,4 +395,4 @@ def send_borrow_dispatch_notify(to_emails: List[str], request_no: str,
 
 此邮件由系统自动发送，请勿回复。
 """
-    send_email(to_emails, subject, content)
+    send_email_async(to_emails, subject, content)