feat(blog): tags 空时从 SEO 关键词自动 fallback 前 3 个

admin 编辑页暂无 tags 输入框，保存的文章 tags 为空，前端卡片无小标签显示
（对比老文章都有"草原穿搭/旅拍穿搭"等 tag）。

在 server/utils/article-helpers.js 加 resolveTagsWithFallback：
- 前端传 tags 数组（非空）→ 按前端
- 否则从 seoKeywords 用 [,，、] 拆分取前 3 个
- 都没有 → []

POST/PUT 均接入。PUT 时用户改 seoKeywords 也会同步刷新 tags。

server/api/admin/articles.post.js

@@ -1,5 +1,6 @@
 import { useDB, schema } from '../../utils/db.js'
 import { requireAdmin } from '../../utils/auth.js'
+import { resolveTagsWithFallback } from '../../utils/article-helpers.js'
 
 export default defineEventHandler(async (event) => {
   requireAdmin(event)
@@ -21,7 +22,7 @@ export default defineEventHandler(async (event) => {
     type: body.type || 'news',
     coverImage: body.coverImage || null,
     author: body.author || '呼籁旅行',
-    tags: JSON.stringify(Array.isArray(body.tags) ? body.tags : []),
+    tags: JSON.stringify(resolveTagsWithFallback(body)),
     seoTitle: body.seoTitle || null,
     seoDesc: body.seoDesc || null,
     seoKeywords: body.seoKeywords || null,

server/api/admin/articles/[id].put.js

@@ -1,6 +1,7 @@
 import { eq } from 'drizzle-orm'
 import { useDB, schema } from '../../../utils/db.js'
 import { requireAdmin } from '../../../utils/auth.js'
+import { resolveTagsWithFallback } from '../../../utils/article-helpers.js'
 
 export default defineEventHandler(async (event) => {
   requireAdmin(event)
@@ -18,10 +19,16 @@ export default defineEventHandler(async (event) => {
   if (body.category !== undefined) updateData.category = body.category
   if (body.coverImage !== undefined) updateData.coverImage = body.coverImage
   if (body.author !== undefined) updateData.author = body.author
-  if (body.tags !== undefined) updateData.tags = JSON.stringify(Array.isArray(body.tags) ? body.tags : [])
   if (body.seoTitle !== undefined) updateData.seoTitle = body.seoTitle
   if (body.seoDesc !== undefined) updateData.seoDesc = body.seoDesc
   if (body.seoKeywords !== undefined) updateData.seoKeywords = body.seoKeywords
+  // tags：前端传了用前端的；没传/空数组时从 seoKeywords fallback（admin 编辑页暂无 tags 输入）
+  if (body.tags !== undefined) {
+    updateData.tags = JSON.stringify(resolveTagsWithFallback(body))
+  } else if (body.seoKeywords !== undefined) {
+    // 用户改了 seoKeywords 但没传 tags，同步更新 tags
+    updateData.tags = JSON.stringify(resolveTagsWithFallback(body))
+  }
   if (body.published !== undefined) updateData.published = body.published ? 1 : 0
 
   db.update(schema.articles)