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/api.go

@@ -57,9 +57,6 @@ func setupAPIRoutes(r *gin.Engine, cfg *config.Config, d *dependencies) {
 	postsAPI.Use(d.authMdw.Required())
 	postsAPI.Use(middleware.CSRF(cfg))
 	{
-		postsAPI.POST("", d.postCtrl.Create)
-		postsAPI.PUT("/:id", d.postCtrl.Update)
-		postsAPI.DELETE("/:id", d.postCtrl.Delete)
 		postsAPI.POST("/:id/submit", d.postCtrl.Submit)
 		postsAPI.POST("/upload-image", d.postCtrl.UploadImage)
 	}