The public REST API accepted inline post media as a free-form array and stored
it verbatim, so a client could create/update a post whose media was a bare
external URL we never hosted. Publishing then depended on that third-party URL
staying alive — when it 404'd (e.g. an image proxy), the post failed across
platforms (Facebook 'unsupported media type', X 'HTTP 404', Instagram 'could
not fetch media').
Inline media URLs on create/update now go through the same download + MIME-
validate + host path as the attach-from-url endpoint (MediaAttacher), so the
stored media always points at our own storage. Items already hosted (carrying a
path) pass through untouched. If any URL can't be fetched the request is
rejected with 422 and nothing is persisted, so a post is never created with
broken media. MCP and the web flow were already safe and are unchanged.
- MediaAttacher: extract fetchToWorkspace() + add resolveInlineMedia()
- Post::allowedMediaTypesFor() so the create flow can compute allowed types
without a persisted post
- API Store/UpdatePostRequest: media.* item rules (mirroring the web; prevents
validated() from stripping hosted-item keys)
- PostController store()/update(): host external media before persisting
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