Trim front-end media embeds

[lylo]

Summary

• Remove lower-use front-end auto-embed modules for Bluesky, GitHub Gists, TIDAL, Checkvist, Strava, and Transistor
• Keep front-end auto-embeds for YouTube, Spotify, Apple Music, Bandcamp, and direct image URLs
• Update media embed docs to mark removed front-end providers as explicit {{ embed ... }} only

Rollout note

This should land after #1032 has been deployed, so existing affected posts/pages can use the new server-side embed path.

Verification

• bin/rails test test/models/media_embed/renderer_test.rb test/integration/custom_tags_rendering_test.rb test/controllers/api/embeds_controller_test.rb
• env RUBOCOP_CACHE_ROOT=tmp/rubocop_cache bundle exec rubocop app/helpers/posts_helper.rb app/models/embed_tag_processor.rb app/models/media_embed/renderer.rb app/models/media_embed/providers.rb app/models/media_embed/providers test/integration/custom_tags_rendering_test.rb test/models/media_embed/renderer_test.rb
• bin/importmap audit