feat(file): add editor img auto upload support

[grtsinry43]

Description

Extend the editor image-upload pipeline:

• FileService now delegates custom uploads to a fixed serverless function (file/editor-image-upload) and supports an optional CDN domain via the existing setCustomDomain. This keeps local/S3 flows intact while letting users implement any upload logic in serverless code.
• Add the corresponding configuration options back in (default values + DTO fields) so the editor image upload settings page explicitly exposes all three modes (self-hosted, S3, serverless), and update the “Editor Image Upload” docs to describe the new behavior.
• Update tests/docs to match (S3 uploader spec ordering, link e2e test now stubs ServerlessService, docs navigation adds the new chapter).

Linked Issues

resolve https://github.com/mx-space/core/issues/2469 https://github.com/mx-space/mx-admin/issues/1001

Additional Context

Custom-upload users must create/enable the file/editor-image-upload function per the new docs; otherwise backend will throw “云函数上传未配置或未启用.” Docs PR lives in docs repo; please publish after merging core (don't forget to change the version in docs).