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_audit_service.go

@@ -2,13 +2,14 @@ package service
 
 import (
 	"strconv"
+	"time"
 
 	"metazone.cc/mce/internal/common"
 	"metazone.cc/mce/internal/model"
 )
 
 // Update 编辑帖子（权限在 controller 层检查）
-func (s *PostService) Update(postID uint, title, body string, visibility, postType, reprintSource, declaration string, reprintProhibited bool, categoryID string) error {
+func (s *PostService) Update(postID uint, title, body string, visibility, postType, reprintSource, declaration string, reprintProhibited bool, categoryID, scheduledAt string) error {
 	post, err := s.findPost(postID)
 	if err != nil {
 		return err
@@ -31,6 +32,13 @@ func (s *PostService) Update(postID uint, title, body string, visibility, postTy
 	} else {
 		post.CategoryID = nil
 	}
+	// 定时发布
+	if t, err := parseScheduledAt(scheduledAt); err == nil {
+		if time.Until(t) < 30*time.Minute {
+			return common.ErrScheduledTooSoon
+		}
+		post.ScheduledAt = &t
+	}
 
 	// 已通过帖子：编辑内容保存为待审修订，不影响当前展示
 	if post.Status == model.PostStatusApproved {
@@ -121,6 +129,10 @@ func (s *PostService) Approve(postID uint) error {
 	}
 	post.Status = model.PostStatusApproved
 	post.RejectReason = ""
+	// 审核晚于定时时间 → 即时发布
+	if post.ScheduledAt != nil && !post.ScheduledAt.After(time.Now()) {
+		post.ScheduledAt = nil
+	}
 	if err := s.repo.Update(post); err != nil {
 		return err
 	}