短文支持链接预览卡片：新增 linkmeta 抓取 + 字符集解码

- internal/linkmeta：取正文第一个 http(s) 链接，解析 og: → twitter: → <title>/
  description 逐级兜底，解 HTML 实体、相对图片补成绝对地址。按对外抓取设防：
  只放行 http(s)、读满 512KB 即停、最多 3 次跳转、整体限时；SSRF 防护做在拨号
  那一刻——解析出 IP 后筛掉回环/私网/链路本地/组播/CGNAT 再直连该 IP，堵住
  DNS 重绑定窗口
- 字符集转换：按「HTTP 头 charset → <meta charset> → UTF-8」解码，gb2312 归一
  成 gbk（超集，按声明解会漏字）；认不出的字符名退回原始字节不报错。为此引入
  golang.org/x/text（官方包，约 +1MB 二进制）。tidy 顺带把 aws-sdk 三个包从
  indirect 修正为直接依赖——storage/r2.go 本来就直接用它们
- posts 加 link_card 列（JSON，沿用 images 的编解码）；admin 保存短文时重算：
  同链接沿用旧卡片不重复打远端，删链接或抓取失败则清空，长文不参与
- 前端 LinkCard 组件接两套 UI 的时间线与详情页，配色写成 vivid 变量优先、
  classic 变量兜底，一份样式两边通用；外链图挂了自动隐藏不留空框
- 卡片用 div[role=link] 而非 <a>（vivid 整条短文包在 RouterLink 里，嵌套 <a>
  非法），点击必须 preventDefault——只 stop 挡不住祖先 <a> 的默认激活行为

backend/go.mod

@@ -3,16 +3,18 @@ module oneblog
 go 1.24
 
 require (
+	github.com/aws/aws-sdk-go-v2 v1.47.1
+	github.com/aws/aws-sdk-go-v2/config v1.33.6
+	github.com/aws/aws-sdk-go-v2/credentials v1.20.6
+	github.com/aws/aws-sdk-go-v2/service/s3 v1.113.4
 	github.com/lib/pq v1.10.9
 	github.com/yuin/goldmark v1.7.13
+	golang.org/x/text v0.21.0
 	modernc.org/sqlite v1.39.0
 )
 
 require (
-	github.com/aws/aws-sdk-go-v2 v1.47.1 // indirect
 	github.com/aws/aws-sdk-go-v2/aws/protocol/eventstream v1.7.20 // indirect
-	github.com/aws/aws-sdk-go-v2/config v1.33.6 // indirect
-	github.com/aws/aws-sdk-go-v2/credentials v1.20.6 // indirect
 	github.com/aws/aws-sdk-go-v2/feature/ec2/imds v1.20.1 // indirect
 	github.com/aws/aws-sdk-go-v2/internal/configsources v1.5.4 // indirect
 	github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 v2.8.4 // indirect
@@ -21,7 +23,6 @@ require (
 	github.com/aws/aws-sdk-go-v2/service/internal/checksum v1.11.5 // indirect
 	github.com/aws/aws-sdk-go-v2/service/internal/presigned-url v1.14.4 // indirect
 	github.com/aws/aws-sdk-go-v2/service/internal/s3shared v1.20.4 // indirect
-	github.com/aws/aws-sdk-go-v2/service/s3 v1.113.4 // indirect
 	github.com/aws/aws-sdk-go-v2/service/signin v1.10.1 // indirect
 	github.com/aws/aws-sdk-go-v2/service/sso v1.38.1 // indirect
 	github.com/aws/aws-sdk-go-v2/service/ssooidc v1.43.1 // indirect