[Feature]: 希望支持自定义添加平台（自定义api发文）

### Problem Description / 问题描述

博客站，或者企业资讯，需要做更新，但是有开发能力，可以自行暴露一条发文api，通过插件发文

### Proposed Solution / 建议的解决方案

增加自定义平台的选择与创建，增加 平台名 及 发文api 的表单，让有开发能力的用户自行接入

### Alternatives Considered / 考虑过的替代方案

_No response_

### Feature Type / 功能类型

New platform support / 新平台支持

### Additional Context / 补充信息

_No response_

### Implementation / 实现意愿

- [ ] I'm willing to submit a PR for this feature / 我愿意为此功能提交 PR
- [ ] I'd prefer to wait for community implementation / 我希望等待社区的实现

No files, tests, or entry points are named. Start by tracing how existing platform choices and publishing requests are represented, then determine where a custom platform name and publishing API can be configured. Done means users can create a custom platform and publish through its API without affecting existing platforms.

Written by the indexing model from the issue text.