feat: 定时发布系统 — Phase 5 完成

- Post ScheduledAt 字段
- Scheduler goroutine 每分钟扫描到期文章
- Create/Update 校验最短 30min 定时
- Approve 审核晚于定时则即时发布
- public 列表+空间页过滤未到期文章
- 写文章页 datetime-local 选择器（min=now+30min）
- AutoMigrate 追加新表

internal/service/post_service.go

@@ -69,7 +69,7 @@ func (s *PostService) findPostWithAuthor(id uint) (*model.Post, error) {
 // Create 创建帖子
 // body 为 Vditor 输出的 Markdown，后端直接存储不做 HTML 消毒
 // MD 是纯文本，无 XSS 注入风险；前端渲染时由 Vditor 转 HTML
-func (s *PostService) Create(userID uint, title, body string, visibility, postType, reprintSource, declaration string, reprintProhibited bool, categoryID string) (*model.Post, error) {
+func (s *PostService) Create(userID uint, title, body string, visibility, postType, reprintSource, declaration string, reprintProhibited bool, categoryID, scheduledAt string) (*model.Post, error) {
 	status := model.PostStatusApproved
 	if s.auditEnabled() {
 		status = model.PostStatusDraft
@@ -93,6 +93,13 @@ func (s *PostService) Create(userID uint, title, body string, visibility, postTy
 		id := uint(cid)
 		post.CategoryID = &id
 	}
+	// 定时发布
+	if t, err := parseScheduledAt(scheduledAt); err == nil {
+		if time.Until(t) < 30*time.Minute {
+			return nil, common.ErrScheduledTooSoon
+		}
+		post.ScheduledAt = &t
+	}
 	if err := s.repo.Create(post); err != nil {
 		return nil, err
 	}
@@ -224,3 +231,15 @@ func (s *PostService) Unpin(postID uint) error {
 	post.PinnedAt = nil
 	return s.repo.Update(post)
 }
+
+// parseScheduledAt 解析 RFC3339 时间，非法格式返回零值+error
+func parseScheduledAt(s string) (time.Time, error) {
+	if s == "" {
+		return time.Time{}, nil
+	}
+	t, err := time.Parse(time.RFC3339, s)
+	if err != nil {
+		return time.Time{}, err
+	}
+	return t, nil
+}