Show SignPresenter playlist media from V2 slides

[jzongker]

SignPresenter playlists listed in B1Admin but showed no media items.

Root cause

SignPresenterProvider.getMessages called GET /content/playlists/:id/messages and skipped any message without a truthy url. That content endpoint only returns templates 1 (image) and 3 (video), and only extracts fieldType image / silentVideo, often returning url: "". Typical playlists use other templates, so browse/getPlaylist/getInstructions came back empty.

The documented player contract is GET /v2/playlists/:id, whose messages carry slides[].type + slides[].data.url.

Changes

• Fetch playlist items from /v2/playlists/:id and map image, video, stream, and media-file web slides to ContentFile (slide.data.url → url/downloadUrl, slide.type → mediaType, message thumbnail, slide seconds, loop).
• Fall back to /content/playlists/:id/messages when V2 is empty/missing, now using url || thumbnail || image.
• Playlist listing via /content/playlists is unchanged. Dropbox/Drive untouched.
• Tests for V2 mapping, non-playable slide skip, gallery multi-slide, and content-API fallback.

All 40 content-providers tests pass.