feat(gerp/mail): 支持用户注册多个企业邮箱，上限 5 个

- MailApply 上限由 1 改 5，超出提示"邮箱数量已达上限"

- MyMailbox 返回 MailboxList 数组（按创建时间倒序），替代单 Mailbox

- ChangePassword/Login 支持可选 MailboxName 指定目标邮箱，缺省取第一个

gvsds/apps/gerp/views/mail.py

@@ -41,6 +41,8 @@ _MAIL_TOKEN_TTL = 24 * 3600  # token 有效期 24 小时
 _MAIL_TOKEN_STORE = {}       # token -> {"email": str, "password": str, "exp": float}
 _MAIL_TOKEN_LOCK = threading.Lock()
 
+_MAILBOX_LIMIT = 5           # 每个用户最多申请的企业邮箱数
+
 
 def _PurgeMailTokens():
     """清理已过期 token。"""
@@ -218,9 +220,9 @@ class MailApplyView(ELTAuthenticatedAPIView):
         if not user_bytes:
             return Response({'code': 1, 'msg': '无法识别当前用户'}, status=401)
 
-        # 每个用户最多一个邮箱
-        if UserMailbox.objects.filter(UserUUID=request.user).exists():
-            return Response({'code': 1, 'msg': '您已拥有邮箱，一个用户只能申请一个'}, status=400)
+        # 每个用户最多申请 _MAILBOX_LIMIT 个邮箱
+        if UserMailbox.objects.filter(UserUUID=request.user).count() >= _MAILBOX_LIMIT:
+            return Response({'code': 1, 'msg': f'邮箱数量已达上限（最多 {_MAILBOX_LIMIT} 个）'}, status=400)
 
         mailbox_name = (request.data.get('MailboxName') or '').strip()
         password = request.data.get('Password') or ''
@@ -291,9 +293,11 @@ class MailChangePasswordView(ELTAuthenticatedAPIView):
     permission_classes = [IsAuthenticated]
 
     def post(self, request):
-        mailbox = UserMailbox.objects.filter(UserUUID=request.user, Status=1).first()
+        mailbox_name = (request.data.get('MailboxName') or '').strip()
+        queryset = UserMailbox.objects.filter(UserUUID=request.user, Status=1)
+        mailbox = queryset.filter(MailboxName=mailbox_name).first() if mailbox_name else queryset.first()
         if mailbox is None:
-            return Response({'code': 1, 'msg': '您尚未申请邮箱'}, status=404)
+            return Response({'code': 1, 'msg': '未找到该邮箱' if mailbox_name else '您尚未申请邮箱'}, status=404)
         # 安全：改密必须先验证当前邮箱密码，防止已登录会话被劫持后直接改密
         old_password = request.data.get('OldPassword') or ''
         if not old_password:
@@ -315,27 +319,23 @@ class MailChangePasswordView(ELTAuthenticatedAPIView):
 
 
 class MailMyMailboxView(ELTAuthenticatedAPIView):
-    """我的邮箱信息 + JMAP 连接配置（供浏览器直连）。"""
+    """我的邮箱列表 + JMAP 连接配置（供浏览器直连）。"""
 
     permission_classes = [IsAuthenticated]
 
     def get(self, request):
-        mailbox = UserMailbox.objects.filter(UserUUID=request.user).first()
         session = StalwartMailManager.SessionConfig()
-        if mailbox is None:
-            return Response({'code': 0, 'data': {'Mailbox': None, 'Session': session}})
-        return Response({
-            'code': 0,
-            'data': {
-                'Mailbox': {
-                    'MailboxName': mailbox.MailboxName,
-                    'QuotaMB': mailbox.QuotaMB,
-                    'Status': mailbox.Status,
-                    'CreateTime': mailbox.CreateTime.isoformat() if mailbox.CreateTime else None,
-                },
-                'Session': session,
-            },
-        })
+        rows = list(UserMailbox.objects.filter(UserUUID=request.user).order_by('-CreateTime'))
+        mailbox_list = [
+            {
+                'MailboxName': m.MailboxName,
+                'QuotaMB': m.QuotaMB,
+                'Status': m.Status,
+                'CreateTime': m.CreateTime.isoformat() if m.CreateTime else None,
+            }
+            for m in rows
+        ]
+        return Response({'code': 0, 'data': {'MailboxList': mailbox_list, 'Session': session}})
 
 
 class MailAdminListView(ELTAuthenticatedAPIView):
@@ -403,16 +403,18 @@ class MailLoginView(ELTAuthenticatedAPIView):
     密码仅在本次请求明文传输一次；后端校验通过后把凭据存入进程内存，
     返回 token，后续 JMAP 调用只携带 token（见 MailJmapView）。
 
-    请求体：Password
+    请求体：Password, MailboxName（可选，多邮箱时指定登录哪个；缺省取第一个）
     响应：{code: 0, data: {Token, MailboxName}}
     """
 
     permission_classes = [IsAuthenticated]
 
     def post(self, request):
-        mailbox = UserMailbox.objects.filter(UserUUID=request.user, Status=1).first()
+        mailbox_name = (request.data.get('MailboxName') or '').strip()
+        queryset = UserMailbox.objects.filter(UserUUID=request.user, Status=1)
+        mailbox = queryset.filter(MailboxName=mailbox_name).first() if mailbox_name else queryset.first()
         if mailbox is None:
-            return Response({'code': 1, 'msg': '您尚未申请邮箱'}, status=404)
+            return Response({'code': 1, 'msg': '未找到该邮箱' if mailbox_name else '您尚未申请邮箱'}, status=404)
         password = request.data.get('Password') or ''
         if not password:
             return Response({'code': 1, 'msg': '请输入邮箱密码'}, status=400)