refactor: 彻底移除 /posts/new 旧编辑器，统一迁移至 /studio/write

- 删除 /posts/new 路由，不再保留任何兼容层
- /posts/:id/edit 改为 301 跳转 /studio/write?id=:id
- 删除 posts/new.html 模板和 editor.js
- 移除 PostController.EditPage/Create/Update/Delete 孤儿方法
- 移除 /api/posts POST/PUT/DELETE 孤儿 API（保留 submit 和 upload-image）
- posts/show.html 编辑链接指向 /studio/write

internal/router/frontend.go

@@ -40,11 +40,10 @@ func setupFrontendRoutes(r *gin.Engine, cfg *config.Config, d *dependencies) {
 
 		// 帖子页面
 		pages.GET("/posts", d.postCtrl.ListPage)
-		pages.GET("/posts/new", d.authMdw.Required(), func(c *gin.Context) {
-			c.Redirect(http.StatusMovedPermanently, "/studio/write")
-		})
 		pages.GET("/posts/:id", d.postCtrl.ShowPage)
-		pages.GET("/posts/:id/edit", d.authMdw.Required(), d.postCtrl.EditPage)
+		pages.GET("/posts/:id/edit", d.authMdw.Required(), func(c *gin.Context) {
+			c.Redirect(http.StatusMovedPermanently, "/studio/write?id="+c.Param("id"))
+		})
 
 		// 创作中心（需登录）
 		studioPages := pages.Group("/studio")