Centralize per-platform PostPlatform.meta validation in PostPlatformMetaRules
(shared by web, REST API and MCP). The API and MCP previously only accepted
aspect_ratio, silently stripping channel_id/board_id/privacy_level and the rest
via validated(), so Discord/Pinterest/TikTok couldn't be configured or published
through those entry points. Now all per-platform meta is accepted and persisted,
required-on-publish is enforced on API update (TikTok privacy, Pinterest board,
Discord channel), and the MCP publish tool guards a post's stored meta before
dispatching. Adds API + MCP tests and a PostPlatform discord() factory state.
bdea56d5e1