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

templates/MetaLab-2026/static/js/studio-editor.js

@@ -98,6 +98,13 @@ document.addEventListener('DOMContentLoaded', () => {
 
     document.getElementById('postForm')?.addEventListener('submit', handleSubmit)
 
+    // 定时发布时间选择器 min 设为 now+30min
+    const scheduledAt = document.getElementById('scheduledAt');
+    if (scheduledAt) {
+        const min = new Date(Date.now() + 30 * 60 * 1000)
+        scheduledAt.setAttribute('min', min.toISOString().slice(0, 16))
+    }
+
     // 类型切换联动：转载→显示来源，原创→显示禁止转载
     const typeRadios = document.querySelectorAll('input[name="post_type"]')
     const reprintSourceArea = document.getElementById('reprintSourceArea')
@@ -192,6 +199,7 @@ async function handleSubmit(e) {
         const reprintProhibited = document.getElementById('reprintProhibited')?.checked || false
         const categoryId = document.getElementById('categoryId')?.value || ''
         const tags = document.getElementById('tagInput')?.value.trim() || ''
+        const scheduledAt = document.getElementById('scheduledAt')?.value || ''
 
         const isEdit = !!postId
         const url = isEdit ? '/api/studio/posts/' + postId : '/api/studio/posts'
@@ -211,6 +219,7 @@ async function handleSubmit(e) {
                 reprint_prohibited: reprintProhibited,
                 category_id: categoryId,
                 tags,
+                scheduled_at: scheduledAt,
             }),
         })
         const data = await resp.json()