feat: 对接稿件审核通知系统 + 消息页面系统消息区域

- 新增通知类型 post_approved / post_rejected
- PostService.Approve/Reject 完成后通知作者（通知失败不影响主流程）
- NotificationService 新增 NotifyPostApproved/NotifyPostRejected 方法
- 消息页面顶部增加系统消息标识和分隔线，之下为全部消息列表
- 稿件审核通过通知卡片点击跳转到 /posts/{id} 页面
- 稿件审核拒绝通知卡片点击跳转到 /studio/posts 创作中心
- 新增 postNotifier 接口遵循 ISP 原则

internal/model/notification.go

@@ -2,8 +2,10 @@ package model
 
 // 消息/通知类型常量
 const (
-	NotifyAuditApproved  = "audit_approved"
-	NotifyAuditRejected  = "audit_rejected"
+	NotifyAuditApproved = "audit_approved"
+	NotifyAuditRejected = "audit_rejected"
+	NotifyPostApproved  = "post_approved"
+	NotifyPostRejected  = "post_rejected"
 )
 
 // Notification 消息/通知模型
@@ -20,8 +22,10 @@ type Notification struct {
 
 // NotifyTypeNames 通知类型 → 中文名
 var NotifyTypeNames = map[string]string{
-	NotifyAuditApproved: "系统通知",
-	NotifyAuditRejected: "系统通知",
+	NotifyAuditApproved: "资料审核",
+	NotifyAuditRejected: "资料审核",
+	NotifyPostApproved:  "稿件审核",
+	NotifyPostRejected:  "稿件审核",
 }
 
 // NotificationListResult 消息列表查询结果