Refuse non-media streams and gate video framing

docs/media.md

@@ -158,7 +158,9 @@ that deletion is irreversible. The delete button is disabled for a 5 second coun
   `  /api/media/<scrape>/<item>/stream`, which serves the local file or re-fetches the source message
   for a fresh signed URL, so expired CDN links still play. The grid prefers the cached still; a
   video without one gets its first frame through the same-origin stream endpoint, which avoids
-  cross-origin blocking and URL expiry. Embed `videoUrl`s (gifv/Tenor mp4s) are stored as
+  cross-origin blocking and URL expiry. Only real video-file URLs are framed; provider pages
+  (YouTube embeds, redirector URLs) are never, and the stream route answers `415` when the
+  upstream is not media — so a pending preview shows a placeholder instead of a browser error. Embed `videoUrl`s (gifv/Tenor mp4s) are stored as
   `play_url` and used through the stream. The full link is a separate button that opens the
   original page with the warning. Length comes from Discord's attachment duration and, for
   downloads, an ffprobe probe. GIF tiles carry a translucent **GIF** label top-left; the cached