A Bluesky post embed is images XOR video, so a post that carries both can't
be published there. Enforce it per selected platform:
- Add ContentType::supportsMixedMedia() (false only for BlueskyPost) and reject
image+video in ContentTypeCompatibleWithMedia when the type forbids it.
- Mirror it client-side (useMediaRules forbidsMixedMedia + usePostCompliance)
so the editor blocks scheduling with an inline reason before submit, like
every other per-platform compatibility check.
- Add the no_mixed_media message in all three locales (en/es/pt-BR).
- Cover the rule and enum, including the GIF-counts-as-image case.
